HTTP Headers

If Unmodified Since

HTTP Header

The HTTP If-Unmodified-Since request header makes the request for the resource conditional.
The server will send the requested resource (or accept it in the case of a POST or another non-safe method) only if the resource on the server has not been modified after the date in the request header.
If the resource has been modified after the specified date, the response will be a 412 Precondition Failed error.

The If-Unmodified-Since header is commonly used in the following situations:

In conjunction with non-safe methods like POST, this header can be used to implement optimistic concurrency control. For example, some wikis reject revisions if the stored document has been modified since it was originally retrieved, thus avoiding conflicts.

Combined with a range request using the Range header, this header can ensure that the requested fragment comes from a document that has not been modified since the specified date.

Header type	Request header
Forbidden request header	No

Syntax

If-Unmodified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

Directives

<day-name>

Specify the day of the week using one of the following case-sensitive options: Mon, Tue, Wed, Thu, Fri, Sat, or Sun.

<day>

Provide the day of the month as a two-digit number, such as “04” or “23”.

<month>

Specify the month using one of the three-letter abbreviations: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, or Dec. These are case-sensitive.

<year>

The four-digit year, such as “1990” or “2016”.

<hour>

The hour in 24-hour format as a two-digit number, e.g., “09” or “23”.

<minute>

The minutes as a two-digit number, e.g., “04” or “59”.

<second>

The seconds as a two-digit number, e.g., “04” or “59”.

GMT

Greenwich Mean Time. HTTP dates are always expressed in GMT rather than local time.

Example

http

If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the If Unmodified Since header. This is particularly useful when you want to test how your application handles conditional requests or cache validation during development or debugging. Steps to Modify the If Unmodified 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-Unmodified-Since.
- In the “Header Value” field, enter the date value you want to test (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 If-Unmodified-Since header into all matching requests, allowing you to simulate conditional requests and test how your server handles updates and concurrency controls.