You can sign up any time, it's just that the challenges start on the 1st or the 16th. You can sign up on the 3rd for a start on the previous 1st (I did), provived you're not trying to get points already credited to your account "grandfathered" into the challenge. BTW, when I did (spousal unit too) the CSR was very firm on the 90-days, not 3-months term.