Skip to main content
apiguide.dev

Request Header

If-Modified-Since

: Tue, 07 Jul 2026 10:00:00 GMT

Makes the request conditional; execution occurs only if the resource has changed since the given date.

DirectionRequest Header
StatusStandardized
Seen with304
Defined inRFC 9110, Section 13.1.3
Updated2026-10-14

What is the If-Modified-Since header?

The If-Modified-Since request header is used to check if a resource has been updated since a specific timestamp.

API Usage & Best Practices

  • Ignored when If-None-Match is present: RFC 9110 requires a recipient to ignore If-Modified-Since if the request also carries If-None-Match, because entity-tag comparison is the more accurate condition. Clients send both only for the sake of older intermediaries that might not implement If-None-Match.
  • Use it when there is no ETag: It validates against Last-Modified, which has one-second resolution, so two changes within the same second are indistinguishable. Prefer ETag where the server can generate one.
  • GET and HEAD only: RFC 9110 requires the field to be ignored on any other method, and when the resource has no modification date.
  • Format requirements: Must contain a valid HTTP date (e.g. GMT format).

Examples

GET /v1/leads HTTP/1.1
If-Modified-Since: Tue, 07 Jul 2026 10:00:00 GMT

Status codes seen with it

1

Sponsors

Support the project