#!/usr/bin/perl -w

use strict;
use Time::Local;


# Get midnight of previous day.  We can't simply subtract 24 hours since
# number of hours in each day might be different due to daylight savings.
sub PreviousDay($)
{
   my ($t) = @_;

   # Assuming that input timestamp is already aligned to midnight,
   # going back 12 hours is guaranteed to put us in the previous day.
   my ($sec, $min, $hour, $mday, $mon, $year) = localtime($t - 43200);
   return timelocal(0, 0, 0, $mday, $mon, $year);
}

# Get first Sunday on or before first day of month.
sub GetMonthStart($)
{
   my ($t) = @_;

   # Go back to first day of the month.
   my ($sec, $min, $hour, $mday, $mon, $year) = localtime($t);
   $t = timelocal(0, 0, 0, 1, $mon, $year);

   # Go back to first Sunday.
   for(;;)
   {
      ($sec, $min, $hour, $mday, $mon, $year, $wday) = localtime($t);
      last if $wday == 0;
      $t = PreviousDay($t);
   }
   return $t;
}