In the absence of an answer from the OP, I've done some important research on frozen food.
In Tesco's, if you buy frozen raw.chicken (pieces, they don't sell whole), there is no 'sell by/use by' but a 'frozen on' date. No recommendation on when it should be eaten by
Some of their frozen chicken products like nuggets have a 'best before'.
So at this point I thought, being charitable to the OP, it could be she bought a frozen chicken from a shop which puts a best before date on them, and then got that confused with sell by/use by, and thought it was a strict date, not vague guidance.
However, very few shops sell whole frozen chickens now. Online Tesco doesn't, Waitrose doesn't, Morrisons doesn't, Asda doesn't, even Iceland doesn't (except for one halal one which I couldn't see any date on the front). I gave up the search there. On short, it's pretty unlikely she bought an already frozen chicken.
So we are back to her not understanding how freezers work.
Ps: I'm ill with a bug, hence over invested and bored.