dt

#!/usr/bin/perl

use Getopt::Long;
use POSIX;

# ---------------------------------------------------------------------------
sub main
{
   my ($delta, $format, $offset);

   GetOptions("-when=s" => $offset,
              "-format=s" => $format);

   if ($format eq "")
   {
      $format = "%a %b %e %H:%M:%S %Y";
   }

   $delta = parse_offset($offset);

   printf("%sn", strftime($format, localtime(time + $delta)));
}

# ---------------------------------------------------------------------------
# offsets to handle:
#    'today'
#    'tomorrow'
#    'yesterday'
#    '(last|next) (week|month|year)'
#    '(+|-)3 (seconds|minutes|hours|days|weeks|months|years)'
#    '(+|-)((HH:)MM:)SS'
sub parse_offset
{
   my ($offset);
   
   ($offset) = @_;

   if ($offset =~ /^s*$/)
   {
      $rval = 0;
   }
   elsif ($offset =~ /tomorrow/)
   {
      $rval = 24 * 3600;
   }
   elsif ($offset =~ /yesterday/)
   {
      $rval = -24 * 3600;
   }
   elsif ($offset =~ /today/)
   {
      $rval = 0;
   }
   elsif ($offset =~ /next/)
   {
      $rval = parse_unit($offset, 1);
   }
   elsif ($offset =~ /last/)
   {
      $rval = -1 * parse_unit($offset, 1);
   }
   elsif ($offset =~ /(second|minute|hour|day|month|year)/)
   {
      ($count) = ($offset =~ /(-?d+)/);
      $rval = $count * parse_unit($offset, $count);
   }
   elsif ($offset =~ /(-|+)?(d+s+)?(dd:)?(dd:)?(dd)/)
   {
      $sign = ;
      $days = ;
      $hours = ;
      $minutes = ;
      $seconds = ;

      $rval = $days;
      $rval = 24*$rval + $hours;
      $rval = 60*$rval + $minutes;
      $rval = 60*$rval + $seconds;
      $rval = -1 * $rval if ($sign eq "-");
   }
   else
   {
      print("Unrecognized offset: '$offset'n");
      exit(1);
   }

   return $rval;
}

# ---------------------------------------------------------------------------
# return the number of seconds represented by the recognized unit
sub parse_unit
{
   my ($offset, $count);

   ($offset, $count) = @_;

   if ($offset =~ /second/)
   {
      $rval = 1;
   }
   elsif ($offset =~ /minute/)
   {
      $rval = 60;
   }
   elsif ($offset =~ /hour/)
   {
      $rval = 60 * 60;
   }
   elsif ($offset =~ /day/)
   {
      $rval = 24 * 60 * 60;
   }
   elsif ($offset =~ /month/)
   {
      $now = time();
      @now = localtime($now);

      @then = @now;
      $then[4] += $count;
      $rval = mktime(@then) - $now;
   }
   elsif ($offset =~ /year/)
   {
      $rval = 365.25 * 24 * 60 * 60;
   }

   return $rval;
}

# ---------------------------------------------------------------------------
main();