When using thawed milk, is it ok to mix one day's milk with another...?

...even if the dates are weeks or months apart? My daughter was unable to eat for a couple of months while she was in the NICU but I pumped. Now I have a freezer FULL of frozen milk in my garage and I'm just wondering if I can just go in and grab a few bottles to thaw, or if I have to be mindful of the dates. I understand to use the oldest first and not to keep it for more than 6 months since it's not in a deep freeze. Thanks!