Crash on broadcast messages, because they don't have a timestamp

When you have an OKC local broadcast, and someone replies, the start of the new message thread contains a copy of your original broadcast. This message doesn't have any date/time associated with it, which breaks the code looking for the timestamp.

This fix identifies broadcast messages, and doesn't try to parse a date/time them.

Alternatives:
1. More error checking in the code looking for the timestamp, and fall back to a bogus date/time only if there's an error.
2. Use an empty message date/time for broadcasts.