sub to_seconds {
  my $time_string = shift;
  my $secs;
  if ($time_string =~ m/(\d+):(\d+):(\d+)\s*(AM|PM)/i) {
    $secs = $1*3600 + $2*60 + $3 + (lc($4) eq 'pm' ? 12*3600 : 0);
  } else {
    warn "unable to parse this time: $time_string\n";
  }
  return $secs; # returns undef if unable to parse string
}