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If-Modified-Since Header

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

Header TypeRequest Header
SpecificationStandard RFC compliant
Associated CodesHTTP 304

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

  • Used as a fallback: Often used in place of ETags when date-based resolution is preferred or when proxy caches do not support ETags.
  • 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

Associated HTTP Status Codes

HTTP 304 — Not Modified

Instructs the client that the cached version of the resource is still valid.

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