The use by date on meat including chicken is just a general guideline that really means that the chicken is just not as fresh as it would be before the use by date.
As long as the chicken has been properly frozen and kept frozen until the use by date or past the use by date the chicken is still very safe to eat.
Just check the chicken for a sour smell or slimy feeling or dull looking texture on the skin of the chicken and if it smells bad or looks discolored or has that slimy feeling then it's bad and should be thrown out.
But if the chicken was frozen until you cooked it then it should still be good and shouldn't have any of those bad qualities to it.
I've kept chicken frozen for up a an entire year past the use by date before and it's still been very good to eat and didn't go bad.
If you keep the chicken thawed though in the refrigerator it will only last a few days before going bad but by freezing the chicken it keeps the bacteria from growing on the chicken.
So the chicken should be good long after the use by date as long as you kept it frozen.