Code should not rely on Date: header to file messages

The design currently trusts the Date: header to provide an accurate date.
This is not advisable, especially for earlier emails, as user devices regularly had bad clocks or wildly incorrect TZ offsets.
Also for messages that are subject to moderation, even an accurate Date header can result in the message being filed in a different month from when it was first seen on the mailing list.

It would be better to file messages using the time at which they arrived or to use times gleaned from the headers.
The Date: header should also be stored in raw form.

At the very least, the code should detect and report dates that are obviously wrong.

Start by locating the message-filing logic that relies on the Date: header and the code that stores message headers. Review how arrival time, header-derived times, raw Date: values, and obviously invalid dates are currently handled. The issue does not choose one replacement strategy, so completion criteria need clarification before implementation.

Written by the indexing model from the issue text.