CO-29 denial code means one thing: the claim arrived after the payer’s deadline. Not a coding error. Not a credentialing issue. A timing problem. And timing problems in billing often mean unrecoverable revenue. That’s what makes this denial different from almost everything else in your AR.
Late filing denials don’t work like other denials. You can’t fix a co 29 denial code the way you fix a missing modifier or a wrong diagnosis code. Appeals rarely succeed. Understanding what causes it, which payers enforce the tightest windows, and exactly how to fight it when you still have a chance, that’s what changes the financial outcome.
This guide covers everything you need: payer-specific timely filing limits, the Medicare distinction between CO-29 and N39011, the PI-29 versus CO-29 confusion that trips up even experienced billers, and the step-by-step resolution process for both commercial and Medicare claims.
What Is the CO-29 Denial Code?
The Official CARC 29 Definition
CO-29 is a Claim Adjustment Reason Code (CARC), specifically CARC 29 under the X12 standard, that appears in the CAS adjustment segment of an 835 ERA when a payer determines the claim was received after the allowable filing deadline. It’s built from two components: the Group Code CO and CARC 29. Both are standardized under the X12 code set, which HIPAA mandates all payers use. This isn’t a Medicare-only code. It shows up on commercial, Medicaid, and Medicare ERAs alike.
The official CARC 29 description reads: “The time limit for filing has expired.” That language appears in the CAS (Claim Adjustment Segment) of the 835 ERA when the payer processes the claim. Your billing software receives it as a $0 payment line, either on the specific service line or at the claim level, with CO-29 as the adjustment reason.
“CO” stands for Contractual Obligation. It tells you immediately who is financially responsible: the provider. For Medicare specifically, CMS prohibits billing the beneficiary for CO-29 write-off amounts, with the exception of applicable deductible or coinsurance. That prohibition isn’t a suggestion. Billing a Medicare patient for a denial code 29 write-off violates program rules and puts the practice at compliance risk.
What CO-29 Looks Like on Your 835 ERA
Here’s where most practices lose money they didn’t have to. Payment posters are trained to focus on paid lines. The $0 lines get skimmed or skipped entirely. CO-29 posts quietly in many practice management systems, closes out cleanly, and disappears from the denial worklist before anyone flags it. The pattern is consistent: Group CO, CARC 29, $0 payment, no worklist alert, no follow-up.
CO-29 rarely appears alone on the remittance. RARC N211 (“Missing or incomplete or invalid occurrence”) and RARC N390 both accompany it on many payer remittances. Noridian, a Medicare Administrative Contractor, specifically pairs CARC 29 with N211 on Medicare timely filing denials. Training your posting team to recognize that combination speeds up identification and gets the denial into the right queue faster.
CO-29 vs. PI-29: Understanding the Difference
CO-29 and PI-29 both carry CARC 29 (timely filing expired), but the group code determines financial responsibility: CO means the provider absorbs the write-off under contract, while PI means the payer initiated the adjustment. That’s the entire distinction, and it changes what you do next.
CO-29 appears when the provider is responsible for the late filing under the terms of their contract. PI-29 appears when the payer initiates the adjustment for a timely filing issue that originates on its side, such as a coordination of benefits determination the payer made late. It’s the same underlying denial co 29 situation in terms of the CARC 29 reason, but the responsible party is different.
Patient billing implications differ between the two. Under CO-29, the provider absorbs the write-off and can’t bill the patient for the contractual adjustment. Under PI-29, patient liability depends on whether the beneficiary received proper notification. That distinction matters for compliance on any claim, and it’s especially critical on Medicare, where the pi 29 denial code description affects how you document the adjustment.
Most billing staff don’t check the group code. They see CARC 29, call it a CO-29, and route it to the write-off queue. Checking whether the group code reads CO or PI before routing takes five seconds. Getting it wrong creates compliance exposure that takes significantly longer to untangle.
CO-29, Occurrence Code 29, and Modifier 29: Clearing Up the Confusion
CO-29 is a payer denial code on the 835 ERA indicating late claim filing. Occurrence Code 29 is a UB-04 claim field indicating accident date. Modifier 29 is a CPT modifier unrelated to denial codes. These three share a number and nothing else. If you received a CO-29 on your remittance, neither occurrence codes nor CPT modifiers are part of that picture.
Occurrence Code 29 appears in fields 31 through 34 of the UB-04 claim form. It documents the date of an accident related to a liability claim. That’s a data entry field on the claim itself, not a payer response code. Finding occurrence code 29 on a UB-04 doesn’t trigger a CO-29 denial and has no connection to one.
Modifier 29 isn’t a standard active CPT modifier in current billing practice. If your team started searching for modifier 29 after seeing a denial co 29 on an ERA, the search is headed in the wrong direction. CO-29 lives entirely on the remittance side. It’s a payer adjustment code, not something that appears on the claim form or relates to procedure coding.
Route each correctly and you’ll save everyone time. Occurrence code 29 questions belong with your clinical documentation or institutional billing team. CPT modifier questions go to your coding department. CO-29 denial questions go straight to your AR follow-up and denial management team. Getting them to the right person on the first pass matters.
Why CO-29 Denials Happen: The Real Root Causes
The Causes Most Billing Teams Already Know
Every CO-29 guide opens the same way: the claim was filed after the deadline. That’s accurate, but “filed late” is the outcome, not the explanation. What caused the late filing is what you actually fix. Late submission is the symptom. The causes below are the disease.
Unsigned notes, incomplete SOAP documentation, and physician chart backlogs drive more CO-29 denials than any other single factor when it comes to the denial code for timely filing. Charge entry waits on documentation. Documentation waits on physicians. Physicians are seeing patients. By the time the chart closes, weeks have passed and that window doesn’t come back.
Claims leave your practice management system and hit a rejection at the clearinghouse. The rejection sits in a queue. Nobody checks it for two weeks. Your 277CA acknowledgment file and 999 acceptance report show the claim never made it to the payer. Your filing window just shrank by 14 days and your co 29 denial code worklist doesn’t reflect it yet.
The Causes Most Billing Teams Miss
Secondary claims have their own timely filing clock, and that’s where assumptions get expensive. Some payers count from the primary EOB date. Others count from the original date of service. When teams assume the secondary clock resets after primary payment posts, they’re often wrong, and that assumption causes CO-29 denials on clean, fully billable claims.
Corrected claims aren’t a fresh start on the timely filing limit. Most payers still measure from the original date of service or the original receipt date. A corrected claim submitted 11 months after service might still land inside the window. One submitted at 13 months won’t. That distinction costs practices money every month.
Provider enrollment delays are a quieter version of the same problem. If a provider’s NPI or PTAN wasn’t active at submission, the payer rejects the claim. Credentialing then burns the TFL in medical billing before the provider can bill a single clean claim. This hits new providers joining established groups most often, and it’s entirely preventable with earlier enrollment tracking.
Institutional claims spanning two fiscal years create a specific CO-29 risk that most teams don’t see coming. The portion of services from the prior fiscal year needs to be billed separately as a timely filing denial code trigger. When teams submit one claim covering both periods, the payer splits them, and the prior-year portion frequently lands outside the filing window.
Wrong payer billed first is the final common miss. While the team identifies the error, contacts the original payer, and resubmits to the correct insurer, the actual timely filing clock keeps running. The correct payer’s deadline doesn’t pause for coordination mistakes. That correction window is part of your filing window whether you realize it or not.
The Medicare Timely Filing Rule That Drives CO-29 Denials
CO-29 for Medicare Part B vs. N39011 for Medicare Part A
Medicare doesn’t apply CO-29 universally across all claim types. For Part B professional claims, MAC systems issue the CO-29 denial code when the 12-month window has passed. For Part A institutional claims covering hospitals, SNFs, and home health agencies, MACs issue N39011 instead. Same underlying problem, different code, different claim type, different billing team affected.
Both CO-29 and N39011 are triggered by the same federal regulation: 42 CFR § 424.44. For services furnished on or after January 1, 2010, Medicare requires the claim to be filed no later than one calendar year from the date of service. No MAC extends this timely filing limit. No contract overrides it. One year is the rule, and it applies the same way across every MAC jurisdiction.
Here’s where most practices get it wrong. Medicare counts the MAC receipt date, not the date you submitted electronically or postmarked the paper claim. Electronic claims received after certain evening or weekend system cutoffs get treated as received the next business day. That gap has caused CO-29 denials on claims that were submitted within the window but received one day past it.
Span-date claims add another layer. For Part A institutional claims, the 12-month period starts from the “Through” date, which is the last day of the service span, not the admission date. For Part B professional claims, the period starts from the “From” date. Institutional billing teams managing long inpatient stays need to track the “Through” date specifically. Counting from admission on a 45-day stay creates a 45-day error in the filing calculation.
The 2026 CMS Update You Need to Know: CR 12909
CMS Transmittal CR 12909, effective late November 2024, clarified something that changes how corrected claims work: there is no longer a separate timely filing period for claim adjustments when the original claim was filed on time. If your original claim cleared the 12-month window and needs a correction, that corrected submission isn’t bound by its own new 12-month deadline. That’s a meaningful operational change, and most billing teams haven’t updated their workflows to reflect it.
CR 12909 also added a 60-day limit specifically for inpatient PPS DRG-increasing adjustments. Those must be submitted within 60 days of the remittance date. Missing that window means the adjustment gets rejected even when the original claim was filed correctly. Hospital billing teams should treat this as a separate deadline that runs parallel to standard denial management timelines.
X12 added RARC N921 effective March 1, 2026, with the definition: “The time limit for filing a reconsideration or appeal has expired.” That’s distinct from CO-29, which signals a missed claim filing deadline. N921 signals that the appeal window itself has closed. When both CO-29 and N921 appear on the same remittance, there’s no remaining path forward. Knowing the difference prevents your team from routing a closed appeal as an open denial.
Medicare’s timely filing rules have layers that most billing teams don’t have time to monitor week to week. CR 12909 alone changed how corrected claims are handled, and N921 is already appearing on 2026 remittances. If your team is managing these changes alongside daily claim volume and patient care, it’s worth talking to someone who tracks regulatory updates as their primary job.
At One O Seven RCM, denial management and AR follow-up are built specifically around catching timely filing issues before they become permanent write-offs. If CO-29 denials are showing up in your aging report, we can take a look.
How CO-29 Differs from Other Common Denial Codes
CO-29 gets mislabeled in denial queues more than almost any other code. Billing teams see a $0 line and assume it’s a duplicate, a contract adjustment, or a data edit. Getting the classification right at the point of posting determines whether that claim gets appealed, corrected, or written off correctly.
Here’s how CO-29 differs from the codes it gets confused with most. Every row below includes a provider action column, because knowing the difference only matters if it changes what you do next.
| Denial Code | Official Meaning | Root Cause | Who Is Responsible | Provider Action |
| CO-29 | Time limit for filing has expired | Claim received after payer deadline | Provider (contractual obligation) | Appeal with proof of timely filing or write off; cannot bill patient |
| CO-18 | Duplicate claim or service | Same claim already adjudicated | Provider or system error | Check claim history; do not resubmit if already paid |
| CO-16 | Claim lacks required information | Missing CPT, ICD, NPI, or data element | Provider | Correct data and resubmit immediately to protect the filing window |
| CO-45 | Charge exceeds fee schedule | Billed above allowable amount | Contractual adjustment | Post write-off; do not balance bill patient |
| CO-22 | This care may be covered by another payer | Coordination of benefits issue | Registration or eligibility error | Fix COB, submit to correct primary, then secondary |
| CO-197 | Precertification or authorization absent | Prior auth not obtained or not linked | Provider | Obtain retro auth if allowed; otherwise appeal or write off |
One timing issue matters more than any other in this table. If your team receives a CO-16 on a claim that’s approaching its timely filing deadline, correct and resubmit that day. Don’t let it sit in a standard correction workflow. A CO-16 that converts to a CO-29 because the team waited two weeks is a fully preventable write-off, and it happens more often than it should.
Among all denial reason codes, CO-29 is uniquely unforgiving because the filing window, once closed, leaves no resubmission path. Most denials have a correction route: fix the modifier, link the authorization, update the patient data. CO-29’s co 29 denial code resolution path is narrow, time-sensitive, and often nonexistent once the deadline has fully passed.
How to Resolve a CO-29 Denial: A Step-by-Step Process
When a co29 denial code lands on your worklist, your first move determines everything that follows. Two paths exist: you have proof the claim was filed on time, or you don’t. Which path you’re on changes every step after this. Pull your submission records before doing anything else.
Step 1: Confirm the Payer’s Timely Filing Rule and Clock Start Date
Pull the payer’s provider manual or the specific plan contract. Confirm whether the filing clock starts on the date of service, the discharge date for inpatient claims, or the primary EOB date for secondary claims. These differ by payer and by plan type. The wrong assumption here sends the entire resolution down the wrong path from the start.
Step 2: Check Your Submission Records Against the Receipt Date
Your practice management system shows when you sent the claim. That’s not the same as when the payer received it. Pull the 277CA or your clearinghouse acceptance report. It timestamps the MAC receipt date, which is the payer’s acknowledgment of receipt. If that date falls within the filing window, you have your proof. If it doesn’t, the claim was genuinely late.
Step 3: Gather Your Proof of Timely Filing Documentation
Payers accept EDI acceptance reports, clearinghouse acceptance timestamps, payer portal confirmation screenshots, and the 277CA file as valid proof of timely filing for a denial code for timely filing appeal. Submission reports alone don’t qualify. A submission report shows you initiated the claim. An EDI acceptance report shows the payer received it. That one distinction is responsible for more failed appeals than any other documentation mistake.
Step 4: File the Appeal with Evidence (Commercial Payers)
For commercial payers, file a formal appeal within the payer’s appeal window, which is typically 30 to 90 days from the denial date. Your timely filing appeal letter should include the denial date, the claim number, the date of service, your proof of timely filing documentation, and a clear explanation of any extenuating circumstance that contributed to the delay. Keep it factual and specific.
Step 5: For Medicare, Pursue a Claim Reopening, Not an Appeal
Medicare CO-29 denials aren’t initial determinations under CMS Pub. 100-04 policy, which means the standard redetermination process doesn’t apply to them. If a valid CMS exception exists, such as an administrative error, retroactive entitlement, or a documented MAC system failure, submit a claim reopening request through your MAC. That’s a separate process from the standard five-level Medicare appeal chain, and routing it incorrectly wastes time you don’t have.
Step 6: Document the Root Cause and Close the Loop
Whether the appeal succeeds or the claim gets written off, document the root cause before closing the denial record. What broke in the workflow? Which step failed? Closing the loop on that question is what prevents the next CO-29 denial from the same cause. Resolution without root cause documentation just clears the queue. It doesn’t fix anything.
Medicare CO-29 Exceptions: When a Claim Reopening Can Save the Revenue
CMS policy under Pub. 100-04, Chapter 1, §70.7 recognizes four specific exceptions to Medicare’s 12-month timely filing rule that allow a claim reopening instead of a write-off. For Medicare CO-29 denials, “final” is usually the right word. But if your situation fits one of these four categories, a claim reopening can still recover the payment.
| CMS Exception | What It Means | Documentation Required |
| Administrative error by CMS or MAC | The MAC or a CMS agent made an error that prevented timely filing | Written MAC acknowledgment of error; system outage records; ticket numbers and screenshots |
| Retroactive Medicare entitlement | Patient was not enrolled in Medicare at time of service but gained retroactive coverage | SSA or CMS entitlement letter showing retroactive effective date |
| Retroactive Medicaid recoupment | State Medicaid agency recouped payment from a dual-eligible patient six or more months after the date of service | State recoupment notice with dates and patient identifiers |
| Retroactive Medicare Advantage disenrollment | Patient was retroactively disenrolled from a Medicare Advantage plan with recoupment six or more months after the date of service | MA plan recoupment letter; CMS disenrollment notice |
If an exception applies, submit a claim reopening rather than a standard appeal. For Part A institutional claims, use a Type of Bill with “Q” in the fourth position (TOB xxQ). For Part B professional claims, follow your MAC’s specific reopening form and submission instructions. Attach every piece of exception documentation before submitting. Missing documentation is the most common reason reopening requests fail, not the underlying exception itself.
When none of the four exceptions fit, the timely filing denial code denial is final. Write it off, document the root cause, and redirect your team’s energy toward the workflow change that prevents the next one. Pursuing a CO-29 with no valid exception is time your AR team spends that won’t produce revenue.
Proof of Timely Filing: What Payers Actually Accept
Most timely filing appeals that could succeed get denied for one reason: the wrong document. A submission report is not proof of timely filing. It shows you initiated the claim from your system. An EDI acceptance report shows the payer acknowledged receipt. That’s the document that wins appeals, and most teams don’t have it ready when they need it.
A valid EDI acceptance report used as proof of timely filing must show the claim was “accepted,” “received,” or “acknowledged” by the payer, include a timestamp that falls within the filing period, and be traceable to the specific patient, date of service, and claim number. A generic batch confirmation doesn’t meet this standard for denial code 29 appeals. Payers will reject it, and most don’t tell you why.
When the acceptance report isn’t available, other documents have worked. A primary EOB showing a prior denial supports secondary claim timely filing appeal letter situations. A rejection letter from another insurer demonstrating the claim was in process elsewhere is also accepted by most payers. Payer portal confirmation screenshots with visible date and time stamps are generally accepted as well.
Submission reports from your practice management system get rejected in almost every POTF appeal. Teams keep submitting them because they’re quick to pull. Going into the clearinghouse portal to retrieve the EDI acceptance report takes an extra step, and under daily billing pressure, that step gets skipped. The result is a failed appeal on a denial that was actually winnable.
The simplest way to win a timely filing appeal is to already have the proof of timely filing saved before the denial arrives. Clearinghouse portals retain acceptance records. Downloading and filing the acceptance report by claim at submission takes 30 seconds. Reconstructing it after a CO-29 denial takes considerably longer, assuming it’s still available.
Timely Filing Limits by Payer: 2026 Reference Table
Most billing teams operate on one timely filing assumption across every co29 denial code scenario they encounter. That assumption costs them. Medicare’s 12-month rule doesn’t apply to Cigna. Aetna’s commercial window doesn’t apply to Medicaid. Running a mixed payer panel on a single mental model is one of the most consistent sources of preventable timely filing limit denials.
| Payer | Original Claim TFL | Corrected / Replacement Claim TFL | Secondary Claim TFL | Key Notes |
| Medicare Part B | 12 months from date of service | No separate TFL limit post-CR 12909 if original was timely | N/A (Medicare is primary for Part B) | MAC receipt date applies, not submission date; 42 CFR § 424.44 |
| Medicare Part A | 12 months from “Through” date | No separate TFL limit post-CR 12909 if original was timely | N/A | N39011 issued instead of CO-29 for Part A claims |
| Medicaid | 90 days to 12 months (varies by state) | Varies by state Medicaid program | Varies | Always verify state-specific rules; some states allow 24 months |
| UnitedHealthcare (UHC) | 90 days to 12 months (plan-dependent) | 12 months from original DOS in most plans | 12 months from primary EOB in most plans | Verify specific plan; employer plans often have shorter windows |
| Cigna | 90 to 180 days (plan-dependent) | 12 months from original DOS | 180 days from primary EOB | Corrected claim TFL for employer plans may differ |
| Aetna | 90 to 365 days (plan-dependent) | 12 months from original DOS | 180 days from primary EOB | Medicare Advantage plans follow Medicare rules |
| Blue Cross Blue Shield | 90 to 365 days (plan-dependent) | 12 months from original DOS | 180 days from primary EOB | BCBS varies significantly by state licensee |
| Humana | 12 months from date of service (most plans) | 12 months from original DOS | 6 months from primary EOB | Humana Medicare Advantage follows Medicare rules |
| Tricare | 12 months from date of service | N/A | 6 months from primary EOB date | Tricare is relatively consistent across plans |
These figures reflect standard published guidelines and common contractual terms as of 2026. Specific plan timely filing limits vary by employer contract, plan type, and state regulations. Always verify the TFL for the specific plan shown on the patient’s insurance card before drawing conclusions for any individual claim.
If your practice sees more than three payers regularly, a payer-specific TFL reference document belongs inside your billing department’s daily workflow. Not filed away. Not in a binder on a shelf. Somewhere your team checks it before flagging a claim as late. The difference between 90 days and 12 months is the difference between a recoverable claim and a written-off one.
Preventing CO-29 Denials Before They Happen
Most billing teams already know what drives CO-29 denial code situations. Claims need to go out fast. Clearinghouse rejections need same-day attention. Secondary claims can’t wait weeks for someone to get to them. The knowledge isn’t the gap in practices that see recurring denial code for timely filing write-offs. The workflow that enforces that knowledge is. Prevention is a design problem.
The Daily and Weekly Controls That Actually Work
Charge entry within 48 hours of service is the single highest-leverage prevention control available. Every day between the date of service and charge entry is a day burned from the filing window. Practices that maintain consistent 48-hour charge entry see meaningfully lower co 29 denial code descriptions in their aging reports. It starts the billing clock immediately and keeps the window from silently shrinking.
Every clearinghouse rejection that sits unreviewed for 24 hours drains the filing window without anyone noticing. A daily target of zero unreviewed clearinghouse rejections sounds aggressive. In practice, billing teams that implement it usually find the daily queue is smaller than they expected. The CO-29 denials that result from ignoring it are larger than expected.
Your team shouldn’t be looking up a payer’s TFL in medical billing while working a denial. That lookup should happen at charge entry, not after the denial arrives. A current payer TFL reference document, accessible inside your practice management system or visible at every billing workstation, eliminates the assumption errors that generate preventable write-offs.
Secondary claim creation should trigger the moment the primary ERA posts, not when payment posting is complete. Those two events can be days apart in an active billing department. Secondary claims that wait for posting completion regularly miss the secondary payer’s timely filing window. The automation trigger point is the operational detail that most secondary claim workflows get wrong.
Daily controls catch most timing issues. A weekly review of unbilled encounters by payer catches what the daily workflow misses. Sort by date of service, filter by payer-specific TFL thresholds, and flag anything approaching 50% of the filing window. That flag needs a same-day owner with authority to act, not a standing agenda item at the next team meeting.
Building these controls into an active billing department while managing daily claim volume, staff turnover, and patient care is a lot to carry simultaneously. Practices that see recurring CO-29 denials usually aren’t making knowledge mistakes. They’re running too many moving parts without enough infrastructure to track all of them consistently.
One O Seven RCM’s revenue cycle management and denial management services are built around catching these issues before they become write-offs. If CO-29 denials are showing up as a recurring line item in your aging report, it’s worth a conversation about what your current workflow is missing.
Frequently Asked Questions About CO-29 Denial Code
What does CO-29 mean on an EOB?
CO-29 on an EOB means the claim was received after the payer’s timely filing deadline. The official CARC 29 description reads: “The time limit for filing has expired.” The Group Code CO indicates a contractual obligation: the provider absorbs the write-off and generally cannot bill the patient for this amount. For Medicare Part A institutional claims, the equivalent code is N39011.
Can a CO-29 denial be appealed?
For commercial payers, a CO-29 denial can be appealed if you have documented proof of timely filing, such as an EDI acceptance report or clearinghouse timestamp showing the payer received the claim within the filing window. For Medicare, CO-29 isn’t appealable through the standard redetermination process. Providers must file a claim reopening under CMS Pub. 100-04, §70.7, and only if one of four CMS-recognized exceptions applies.
What is the difference between CO-29 and N39011?
CO-29 and N39011 are both Medicare timely filing denial codes, but they apply to different claim types. CO-29 appears on Medicare Part B professional claims when the 12-month filing deadline under 42 CFR § 424.44 has passed. N39011 is issued by Medicare Administrative Contractors for Part A institutional claims, including hospitals, skilled nursing facilities, and home health agencies, for the same timely filing violation.
What is the difference between CO-29 and PI-29?
CO-29 and PI-29 both carry CARC 29 (the time limit for filing has expired), but the group code determines financial responsibility. CO means Contractual Obligation: the provider is responsible for the adjustment under the contract and typically can’t bill the patient. PI means Payer Initiated: the payer is taking the adjustment, and provider write-off obligations differ. Always check the group code before routing any pi 29 denial code to the write-off queue.
What is proof of timely filing in medical billing?
Proof of timely filing (POTF) is documentation showing a claim was submitted within the payer’s filing deadline. Accepted documents include EDI acceptance reports (which confirm the payer received the claim), clearinghouse acceptance timestamps, 277CA acknowledgment files, and payer portal confirmation screenshots. A submission report from your practice management system alone isn’t accepted by most payers as valid proof of timely filing for a timely filing denial code appeal.
What is the Medicare timely filing limit for CO-29 denials?
Medicare’s timely filing limit is 12 months (one calendar year) from the date of service for Part B professional claims, established under 42 CFR § 424.44. For Part A institutional claims, the same 12-month period applies from the “Through” date of service. Medicare uses the MAC receipt date, not the submission date, to determine timeliness. Claims received even one day past the 12-month window are denied with CO-29 for Part B or N39011 for Part A.