I would create a table with a key of user id and date with a login flag (0 means no login, 1 means one or more logins for that user on that date).
Using a table (Data) containing user ids and actial login dates, you could use something like this to create a login flag table:
Date(Floor(date)) As date,
1 As login_flag
How you use depends on details of your data model.
Hope that helps