HTTP headers / request header
If-Modified-Since: Tue, 10 Jun 2026 18:31:00 GMT
Conditional GET by date: unchanged since this timestamp → 304.
Ignored when If-None-Match is also present; the date must be in RFC 7231 GMT format or servers skip it silently.
The fastest way to see what If-Modified-Since is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · Authorization · Accept · Accept-Encoding · Accept-Language · If-None-Match · If-Match
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.