The email date field specifies the date and time the email was sent, offset from Greenwich Mean Time, similar to the following:
Wed, 17 Jan 2000 16:40:10 -0500
This time is specified in 24-hour format, so that 16:40 is the same as 4:40 PM. The number at the end, “-0500”, is an offset added to each email by the originating email server to indicate the number of hours difference between local time and the world baseline Greenwich Mean Time (GMT) in Greenwich, England. Therefore, an email with an offset of -0500 is five time zones west of Greenwich, and an email with an offset of +0400 is four time zones east of Greenwich. GMT is also sometimes referred to as Universal Time Coordination (UTC), and sometimes as Zulu time, and has been based on atomic clocks since 1986.
The GMT offset can be used with a little calculation to compare the times of emails sent from different places around the world. This can be useful when reviewing late breaking news stories, or following newsgroup postings, to see which was sent first.
Remember that on the Internet that a person may be physically in one time zone and logging into an email server in a different time zone, so their email will be labeled with a time and GMT offset that reflects their email server’s location, which may not necessarily be their own location.