Frozen breastmilk

Okay so I have a very small freezer stash of bm. I was freezing it when my baby was a week or 2 old, so I was only getting 1 or 2 oz a pumping session. I go back to work tomorrow 😭💔 im wondering if I can combine 2 different containers of frozen breastmilk...? Baby is 3 months & obviously drinking more than 1oz a session now. Do they have to be from the same day/pumping session if I combine them? Or does anyone know a website that would have this info?