What is an If-Unmodified-Since header?
Makes the request conditional on the resource not having changed since the given date.
If-Unmodified-Since is the inverse of If-Modified-Since - instead of asking for a fresh copy only if something changed, it tells a website's server to reject the request outright if the resource has changed since a given time, which is handy for safely resuming an upload or edit without accidentally clobbering someone else's more recent changes.
Common use case
A resumable upload client includes If-Unmodified-Since with the timestamp it last synced, so the server rejects the continuation with 412 if the file was modified elsewhere in the meantime.
Example
If-Unmodified-Since: Sat, 22 Aug 2026 09:15:00 GMTHistory
Part of the original HTTP/1.1 specification, RFC 2068 (1997).
Did you know?
It's the timestamp-based counterpart to If-Match, playing the same lost-update-prevention role but comparing dates instead of ETags.