Treat absent due dates as infinitely in the future

The simplest way to achieve my desired effect of sorting tasks without due dates after those which are deadlined.  I don't know if it's just me personally, but I find this makes a lot more sense. :)  (And of course, the behavior anywhere without a default date supplied remains unchanged.)

Note I chose 2038-01-17 as it is the maximum timestamp that can fit in 31 bits (minus more than 24 hours to be extra safe).  There's probably a way to find out how wide timestamps are in the current Perl interpretor and to derive a date from that, but this is already good enough for the next 23 years.