my $start = $format->parse_datetime(...);
my $stop  = $format->parse_datetime(...);

$start = $min_start if $start < $min_start;
$stop  = $max_stop  if $stop >= $max_stop;

my $dur = $start->delta_ms($stop);

printf("%d minutes and %d seconds\n", $dur->in_units(qw( minutes seconds ));