I've modified grepmail to match the date against any of the three dates in the header, rather than just the first recognizable one. Also, I've made it accept dates in unixtime format.
The reason for this is I've extracted a list of messages flagged 'important' from the Evolution email client, but the only way to match the list to the message is by date, msg-from and msg-to. Date is provided in unixtime format. Rather than convert the time twice, it was easier to modify grepmail. Also, the time reported is from the '^From' line, not the 'Date:' line.