As a new TM customer, do I have to start using the benefits right away, or could I start them a few weeks or months later?

For example, the AppleTV+ benefit… if I wanted to wait until my current subscription ends at the end of the month, would I still be able to sign up and get the full 6-month promo from T-Mobile, or would it only be 5-months since I waited a month to sign up for the benefit?