HTTP headers
If-Modified-Since HTTP Header Meaning
If-Modified-Since asks whether a resource changed after a timestamp.
Meaning
The client is validating cached content using Last-Modified time.
Where it appears
Browsers send it for conditional requests when a timestamp validator is available.
What to check
Check server clock behavior and prefer ETag validation when precise version comparison matters.
Example
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT