Store dates internally as "seconds since start of epoch". That way you can easily convert from one format to another.
When showing dates (on a screen, or a printed report) DO NOT invite confusion between US and non-Us people by the ambiguous m/d/yyyy (or was that d/m/yyyy) format. Use the three character abbreviation for the month. So the date would be shown as Jul 4 2001 (or 7 Apr 2001).