Excellent post. Just one point which I would pick up on, is that values for the month, weekday and day of the year passed to POSIX::mktime begin at zero. As such, the determination of $then_time in your post should alternatively read:

my $then_time = POSIX::mktime
(
    0,                      #   Seconds
    0,                      #   Minutes
    0,                      #   Hours
    $date_parts[1],         #   Day (of month)
    $date_parts[0] - 1,     #   Month
    $date_parts[2] - 1900   #   Year
);
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This behaviour is documented on the POSIX module man page.

 

perl -le 'print+unpack"N",pack"B32","00000000000000000000001001100100"'