Proof of timely filing is the evidence that a claim was accepted by (or on its way to) the payer within the filing window. The strongest proof is an electronic acceptance record — a 277CA accepted-claim acknowledgment or a clearinghouse acceptance report — tying the specific claim to a date inside the window. A billing-system screenshot is weak by comparison because it shows intent, not receipt.
What counts as proof — strongest first
- A 277CA (claim acknowledgment) showing the claim was accepted within the window — the gold standard.
- A clearinghouse acceptance/transmission report identifying the claim and date.
- Payer correspondence or a portal record confirming earlier receipt.
- Evidence the delay was payer-caused (wrong COB, an incorrect denial you reworked) as good cause.
Why a rejection is not proof
A clearinghouse rejection means the claim never reached the payer — so it does not establish timely filing. Only an accepted/acknowledged submission counts. This is the single most common reason a timely-filing appeal fails: the provider files within the window, the claim rejects, and the corrected claim arrives after the deadline.
Accepted ≠ submitted
Frequently asked
Is a screenshot of my billing system enough?
What if the payer caused the delay?
Primary sources: 42 CFR 424.44 (Medicare time limits for filing claims). General information, not legal or medical advice — confirm against the governing rule for the plan type.
When the appeal has to be written, and cited
Upload the denied EOB and Merits returns a complete, citation-verified appeal letter — the clinical argument, the payer's own coverage criteria, and your federal appeal rights — in about a minute.