What is valid RFC1123 date format

Question

I am working on a service which returns expires header. The service will be working across different timezone. So we need a way to return other timezones than GMT.

I know that the http header have to follow RFC1123 standard date format. So the service returns date like below -

Fri, 01 Mar 2019 15:00:00 GMT

What I need is return the date in below format.

Fri, 01 Mar 2019 15:00:00 +0530

Is this a valid date for RFC1123 date format?

Solution

I guess you are using the old and obsolete RFC 2616 as reference. Please bear in mind that such document is no longer relevant nowadays and has been replaced with the following documents:

• RFC 7230: Message Syntax and Routing
• RFC 7231: Semantics and Content
• RFC 7232: Conditional Requests
• RFC 7233: Range Requests
• RFC 7234: Caching
• RFC 7235: Authentication

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According to the RFC 7231, HTTP dates must be expressed in GMT. So expressing dates with offsets from UTC don't seem to be valid valid.

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See how the Expires header is defined in the RFC 7234:

5.3. Expires

The Expires header field gives the date/time after which the response is considered stale. [...]

The Expires value is an HTTP-date timestamp, as defined in Section 7.1.1.1 of RFC 7231.

 Expires = HTTP-date

For example

Expires: Thu, 01 Dec 1994 16:00:00 GMT

Now see the following quote from the RFC 7231:

7.1.1.1. Date/Time Formats

Prior to 1995, there were three different formats commonly used by servers to communicate timestamps. For compatibility with old implementations, all three are defined here. The preferred format is a fixed-length and single-zone subset of the date and time specification used by the Internet Message Format [RFC5322].

HTTP-date    = IMF-fixdate / obs-date

An example of the preferred format is

Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate

Examples of the two obsolete formats are

Sunday, 06-Nov-94 08:49:37 GMT   ; obsolete RFC 850 format
Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format

A recipient that parses a timestamp value in an HTTP header field MUST accept all three HTTP-date formats. When a sender generates a header field that contains one or more timestamps defined as HTTP-date, the sender MUST generate those timestamps in the IMF-fixdate format.

An HTTP-date value represents time as an instance of Coordinated Universal Time (UTC). The first two formats indicate UTC by the three-letter abbreviation for Greenwich Mean Time, GMT, a predecessor of the UTC name; values in the asctime format are assumed to be in UTC. [...]