We manage a wait list for membership so I would like to know if there is a way to calculate a number for a custom filed that would be based on if a member is a subtype and then the date in another custom filed. So that we would have a position number every time we entered a contact to the subtype waitlist. and would recalculate all the numbers after a contact was removed from the waitlist subtype
Sorry - there is no membership wait list concept in CiviCRM. To achieve what you describe here will require custom code -> a CiviCRM extension.
Ditto to KarinG. A way around this is to build some sort of report with all the contacts/members. Then sort the report with last created or last modified date. You won't be able to get a position #, but you at least have the order of the waitlist.