calculate number of days between 2 dates (without external modules)

Problem illustration:
All inputs in format 'dd/mm/yyyy'

sample input A:
$date1 = '21/10/2004';
$date2 = '25/10/2004';

sample output A:
4

sample input B:
$date1 = '21/10/2004';
$date2 = '21/10/2005';

sample output B:
365


sample input C:
$date1 = '21/10/2004';
$date2 = '21/10/2020';

sample output C:
5844

Without using third party modules (Date::Calc etc.)
Must calculate correctly with leap years, etc.
Does this sound like reinventing the wheel?

Opinions welcome.