If-Modified-Since
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The HTTP If-Modified-Since
request header makes a request conditional.
The server sends back the requested resource, with a 200
status, only if it has been modified after the date in the If-Modified-Since
header.
If the resource has not been modified since, the response is a 304
without any body, and the Last-Modified
response header of the previous request contains the date of the last modification.
Unlike If-Unmodified-Since
, If-Modified-Since
can only be used with a GET
or HEAD
.
When used in combination with If-None-Match
, it is ignored, unless the server doesn't support If-None-Match
.
The most common use case is to update a cached entity that has no associated ETag
.
Header type | Request header |
---|---|
Forbidden header name | No |
Syntax
If-Modified-Since: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT
Directives
<day-name>
-
One of
Mon
,Tue
,Wed
,Thu
,Fri
,Sat
, orSun
(case-sensitive). <day>
-
2 digit day number, e.g., "04" or "23".
<month>
-
One of
Jan
,Feb
,Mar
,Apr
,May
,Jun
,Jul
,Aug
,Sep
,Oct
,Nov
,Dec
(case sensitive). <year>
-
4 digit year number, e.g., "1990" or "2016".
<hour>
-
2 digit hour number, e.g., "09" or "23".
<minute>
-
2 digit minute number, e.g., "04" or "59".
<second>
-
2 digit second number, e.g., "04" or "59".
- GMT
-
Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.
Examples
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
Specifications
Specification |
---|
HTTP Semantics # field.if-modified-since |
Browser compatibility
BCD tables only load in the browser
See also
ETag
If-Match
,If-None-Match
,If-Unmodified-Since
conditional request headers304 Not Modified
,412 Precondition Failed
response status codes