# If-Modified-Since header > Makes the request conditional; execution occurs only if the resource has changed since the given date. Source: https://apiguide.dev/headers/if-modified-since Direction: request Registered: yes Defined in: RFC 9110, Section 13.1.3 Updated: 2026-10-14 ## 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`](/headers/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`](/headers/last-modified), which has one-second resolution, so two changes within the same second are indistinguishable. Prefer [`ETag`](/headers/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 ```http GET /v1/leads HTTP/1.1 If-Modified-Since: Tue, 07 Jul 2026 10:00:00 GMT ```