Caveat: What Unit's can do versus what they are willing to do or usually do differs, check with your Unit folks for local policies.
Normally for the reserves, your UTA's start the day you are gained if you come in off the street or from active duty. So if you come in halfway through the year you have half left. Also once a UTA is passed you can't reach back for it. So if you are going on orders that cover a UTA make sure to reschedule them prior to the UTA date sometime into the future or you'll be counted constructively present and not be able to use it later. AFTP's you still can theoretically do all 48 as long as you don't exceed 16 per quarter. So of you come in the third quarter you have 32 available for the rest of the FY. Different Unit CC's have different policies regarding UTA reschedules and I have had one that wouldn't let you reschedule a UTA if you were on orders those dates. That's rare but can happen. Most want you to have max availability so they'll punt your UTA's forward. Especially with today's lack of RPA funding they're screwing themselves by not letting you do so. But its up to you to make sure it gets done cause once that original UTA date goes by, there needs to be a 40A in the system or it goes bye bye. Also some unit's have mandatory UTA's, a couple a year that you can't reschedule and are for big max training events like water survival or block training.