While you see it as “day 1”, what happens when you reach “day 101” then? You would have to programmatically change the starting date.
But when dealing with databases, you can do a query for between two dates. So for a very streamlined setup, you would do current_date and current_date-100 days and then do a count of days active that is returned. The 100 days is a variable that they can simply change to resize the timeframe.
As for the current prerequisites:
Sometimes you just might run across someone who has knowledge of something can enlighten yourself by questioning what they say. The seeker of knowledge is the one who asks questions.