Your $_time_now is a float, in minutes. You could multiply its fractional part by 60 to get the seconds back, but here is another way which works on elapsed seconds, ($_current_time - $_session_time),

sub fmin_sec {
    my $sec = shift;
    sprintf '%01d:%02d', $sec / 60, $sec % 60;
}
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After Compline,
Zaxo