← Back to headers
HTTP Header
If-Modified-Since
CachingMakes the request conditional on the resource being modified after the given date.
HTTP header reference, syntax, examples, and developer usage.
What is the If-Modified-Since header?
The If-Modified-Since HTTP header is used to transmit metadata between a client and server as part of HTTP requests or responses.
HTTP headers define how content should be interpreted, cached, authenticated, secured, or processed by browsers and APIs.
Direction
This header may appear in both HTTP requests and responses.
Syntax
If-Modified-Since: <http-date>
Example
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
Common use cases
- Conditional GET requests
- Browser caching
- Returning 304 Not Modified
Common mistakes
- Using the header in the wrong request or response context
- Sending invalid header values
- Incorrect header syntax
- Assuming the header automatically changes server behaviour