HTTP Headers
If Modified Since
HTTP Header
The HTTP If-Modified-Since request header makes a request conditional.
The server responds with the requested resource and a 200 status only if the resource has been modified after the date specified in the If-Modified-Since header.
If the resource has not been modified since then, the server returns a 304 status without any body, and the Last-Modified response header of the previous request contains the date of the last modification.
Note: The Last-Modified header helps clients determine if the resource has changed since their last fetch.
Unlike If-Unmodified-Since, the If-Modified-Since header can only be used with GET or HEAD requests.
When combined with If-None-Match, it is ignored unless the server does not support If-None-Match.
This header is mainly used to update cached entities that do not have an associated ETag header, reducing unnecessary data transfer when content hasn’t changed.
The most common use case is updating a cached copy of a resource that lacks an ETag.
Syntax
If-Modified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
Directives
Directives
<day-name>
Specify the day of the week using a case-sensitive string: “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”, or “Sun”.
<day>
Use a two-digit number to denote the day of the month, such as “04” or “23”.
<month>
Specify the month by its abbreviation with case sensitivity: “Jan”, “Feb”, “Mar”, “Apr”, “May”, “Jun”, “Jul”, “Aug”, “Sep”, “Oct”, “Nov”, or “Dec”.
<year>
Use a four-digit year number, such as “1990” or “2016”.
<hour>
Specify the hour with a two-digit number in 24-hour format, e.g., “09” or “23”.
<minute>
Use a two-digit number to indicate minutes, such as “04” or “59”.
<second>
Use a two-digit number to represent seconds, such as “04” or “59”.
GMT
Greenwich Mean Time. HTTP dates are always expressed in GMT and never in local time.
Example
http
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMTHow to Modify Header using Requestly
Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the If-Modified-Since header. This is especially helpful for testing how your application handles caching and conditional requests during development or debugging. Steps to Modify the If-Modified-Since Header:
- Install and open the Requestly Chrome extension. You can find it on the Chrome Web Store.
- Create a new rule: Click on “Create Rule” and choose “Modify Headers” from the list of available rule types.
- Add a new header modification:
- Under “Action”, select “Add” or “Override”.
- In the “Header Name” field, enter If-Modified-Since.
- In the “Header Value” field, enter the desired date and time (e.g., Wed, 21 Oct 2015 07:28:00 GMT).
- Set the URL condition: Specify the URL or pattern where this header change should apply (e.g., https://your-api.com/*).
- Save the rule.
Once set up, Requestly will inject the specified If-Modified-Since header into all matching requests, allowing you to test how your server responds to conditional GET requests and validate cache behavior.
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