Proper Mail Date Header Formatting (RFC 4021, RFC 2822, RFC 822) and Analysis of 132k Date Headers

Emails generated by our applications were displaying universal time instead of a user’s local time when viewed in Thunderbird.  Our servers use universal time.  The emails we send to our users include a date header with UTC.  Other email clients automatically convert this to the user’s local time.

I compared the date header in our messages to date headers in other messages in my inbox.  I found that there were a variety of formats and wasn’t sure which one was correct, so I turned to the RFCs.  I found RFC 4021 first, which referred to RFC 2822 and RFC 822.  The proper “date-time” syntax was originally defined in RFC 822 section 5.1. and later clarified in RFC 2822 section 3.3.

Even the clarifications in RFC 2822 allowed several different formats, so I analyzed 132,037 date headers on one of our mail servers, hoping to determine if a specific format was most common.  I found that the following format is by far the most common:

Date: Tue, 18 Nov 2014 15:57:11 +0000

 

The day of month and hour of day are both two digits.  The time zone is a 4-digit offset prefixed with either a “+” or “-“.  In the example above, the server is set to GMT or UTC, so the offset is “+0000”.  Be aware that “+0000” and “-0000” are not handled the same!  RFC 2822 section 3.3 says that offset “-0000” should be treated as an unknown timezone.

The following PHP code will output the format above:

echo "Date: " . date("D, d M Y H:i:s O");

It also seems acceptable to place the time zone or a comment in parenthesis after the date.  Here are a few examples:

Date: Fri, 03 Dec 2010 16:02:30 -0600 (CST)
Date: Fri, 9 Sep 2005 16:38:47 -0400 (added by postmaster@attrh1i.attrh.att.com)

I have attached the data file that contains 130,606 date headers (after removing 2k of mangled records that included other email contents) in case you would like to perform additional analysis.

smtp-header-dates-20141117.txt

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