sub is_leap_year
{
    my $year = shift;   #  We assume that validation has taken place.
    if    ( $year % 400 == 0 ) { return 1; }
    elsif ( $year % 100 == 0 ) { return 0; }
    elsif ( $year %   4 == 0 ) { return 1; }
    else                       { return 0; }
}