I usually make 2 functions 1 to convert to unix time the other to return a formated string. I usually prefer to avoid POSIX calls since that won't work on Windows. So I prefer to simply use Time::Local.

To get the difference between 2 dates simply convert them both to seconds and then use some subtraction magic.

use Time::Local;

my %months = (
  'Jan' => 0,
  'Feb' => 1,
  'Mar' => 2,
  'Apr' => 3,
  'May' => 4,
  'Jun' => 5,
  'Jul' => 6,
  'Aug' => 7,
  'Sep' => 8,
  'Oct' => 9,
  'Nov' => 10,
  'Dec' => 11
);

my $date = '2001-01-01';
my $nodays = int((get_unixtime($date) - time) / 60 / 60 / 24);

printf "No days: $nodays\n";

# Convert date to number of seconds from 1970
sub get_unixtime {
  my $date = shift;

# MM/DD/YYYY
  if($date =~ /(\d+)\/(\d+)\/(\d\d\d\d)/) {
    return timegm(0, 0, 0, $2, $1 - 1, $3);
  }
# D MMM YYYY
  elsif($date =~ /(\d+) (\w\w\w) (\d\d\d\d)/) {
    return timegm(0, 0, 0, $1, $months{$2}, $3);
  }
# YYYY-MM-DD
  elsif($date =~ /(\d\d\d\d)-(\d\d)-(\d\d)/) {
    return timegm(0, 0, 0, $3, $2 - 1, $1);
  }

  return 0;
}

# Convert number of seconds since 1970 to YYYY-MM-DD
sub get_date {
  my $sec = shift;
  my @time = gmtime($sec);
  return sprintf "%04d-%02d-%02d", $time[5] + 1900, $time[4] + 1, $tim
+e[3];
}
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