The schedule is planned a long ways ahead. However, the schedule is also subject to change.
For example, I was supposed to have drill next weekend, and AT the first two weeks of June. About two weeks ago, we had been informed that there was something of a scheduling cock-up, and drill is now end April, and AT covers the last two weeks in June, early July. This is a big thing, not least because my wedding now conflicts with AT.
I am not going to reschedule my wedding.
I wouldn't be too worried if the employee does this only one month a year or so. Anything more than that, and you'll be best served calling the unit and speaking to the full-timer to just verify that it was indeed a recent change.
[ETA: Even if the Guardsman is dropping the ball, and not lying about the weekends, but just not arsing himself to be polite enough to let the employer know about it, you can't legally repremand him for it. However, you can call his commander, and advise him of the soldier's failings, the military will take care of it]
NTM