Can a police department postpone hiring me until after national guard annual training?

I was hired by a police department recently and was given a date for the academy. I told them I had my AT from late April to late May, but they scheduled me to attend in April. I asked and I need to go to my AT so now the department is saying they will need to push back my start date until after I get back from AT. Usually before attending the academy there is some on the job training, uniform issue etc. Is it legal for them to push that start date back? I still have the job just will have to start 3 months after what they initially told me. Just trying to get a better picture, thanks