The HTTP If-Modified-Since request header is used to transform the HTTP request into one that is conditional based on whether it has been modified since a specific date and time.
The If-Modified-Since request header is included by the client to indicate that the server is only to complete the HTTP request if the target resource has been modified since the specified date and time. Unlike the companion method, HTTP If-Unmodified-Since, this HTTP header can only be used for HTTP GET and HTTP HEAD requests.
A common use of If-Modified-Since is to refresh a cache object that does not have an ETag.
The directives are day-name, day, month, year, hour, minute, second, and GMT.
For a complete description of the directives, please refer to the HTTP Date header.
In this example, the HTTP request will not be fulfilled unless the resource has been modified since June 1st, 2022, at 8 am GMT.
If-Modified-Since: Wed, 01 Jun 2022 08:00:00 GMT
The If-Modified-Since header is used to transform the HTTP request into a conditional request, where the resource must be modified on or beyond a specific date for the HTTP request to proceed.