The first one is correct. In this sentence, "just" means "just now". Even though the first part of your sentence is using the past tense (noticed), "just" gives it a present feel. So you need to use "is" instead of "was". I just (now) noticed it is 2am. (this is happening in the present) I was watching TV and I then I noticed it was 2am. (this is all happening in the past)

The correct one is "I just noticed it IS 2 a.m." Using "just" implies current action. Without "just", either sentence could be correct, depending on the context.