error.

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

  * In conjunction with non- [ safe ](/en-US/docs/Glossary/Safe/HTTP) methods, like [ ` POST ` ](/en-US/docs/Web/HTTP/Methods/POST) , this header can be used to implement an [ optimistic concurrency control ](https://en.wikipedia.org/wiki/Optimistic_concurrency_control) , as is done by some wikis: editions are rejected if the stored document has been modified since the original was retrieved. 
  * In conjunction with a range request using the [ ` Range ` ](/en-US/docs/Web/HTTP/Headers/Range) header, this header can be used to ensure that the new fragment requested comes from an unmodified document. 

Header type  |  [ Request header ](/en-US/docs/Glossary/Request_header)  
---|---  
[ Forbidden header name ](/en-US/docs/Glossary/Forbidden_header_name) |  no   
  
##  Syntax

http

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

##  Directives

<day-name>

    

A 3-letter description of the day of the week. One of "Mon", "Tue", "Wed",
"Thu", "Fri", "Sat", or "Sun" (case-sensitive).

<day>

    

A 2-digit day number of the month. Examples: "04", "23".

<month>

    

A 3-letter description of the month. One of "Jan", "Feb", "Mar", "Apr", "May",
"Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case-sensitive).

<year>

    

A 4-digit year number. Examples: "1990", "2016".

<hour>

    

A 2-digit hour number based on a 24-hour system. Examples: "09", "23".

<minute>

    

A 2-digit minute number. Examples: "04", "59".

<second>

    

A 2-digit second number. Examples: "04", "59".

` GMT `

    

Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local
time.

##  Examples

http

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

##  Specifications

Specification  
---  
[ HTTP Semantics  
#  field.if-unmodified-since  ](https://www.rfc-
editor.org/rfc/rfc9110#field.if-unmodified-since)  
  
##  Browser compatibility

BCD tables only load in the browser  with JavaScript enabled. Enable
JavaScript to view data.

##  See also

  * [ ` Last-Modified ` ](/en-US/docs/Web/HTTP/Headers/Last-Modified)
  * [ ` If-Modified-Since ` ](/en-US/docs/Web/HTTP/Headers/If-Modified-Since)
  * [ ` If-Match ` ](/en-US/docs/Web/HTTP/Headers/If-Match)
  * [ ` If-None-Match ` ](/en-US/docs/Web/HTTP/Headers/If-None-Match)
  * [ ` Range ` ](/en-US/docs/Web/HTTP/Headers/Range)
  * [ ` 412 Precondition Failed ` ](/en-US/docs/Web/HTTP/Status/412)

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