Invalid Date header in Emails

Emails sent by phpBB3 specify the Date header in UTC and actually use the abbreviation "UTC" as the timezone designator. Unfortunately, this isn't valid according to RFC 2822. As a result, recipients' email software tends to misinterpret the date/time; e.g., Thunderbird thinks it's in the recipient's local timezone instead of UTC. The correct timezone designator for UTC, according to RFC 2822, is +0000 (the abbreviation "UT" would also work but is considered obsolete).
Note: instead of simply fixing this to indicate dates correctly in UTC, my preference would be the method used by phpBB2, where Date headers are specified in the server's local timezone - as I find this very helpful in matching emails with events in the server log.