Unless is "A menos que". In this sentence it would be "A menos que no cuentas anoche." Technically this isn't a complete sentence because there is no subject, but it's just an example of how you could add this phrase to your vocabulary. An example would be: I haven't smoked at all this week, if you don't count last night.
Because cuentas is the tu form of the verb contar. The subject "you" is what is doing the counting, not the night. Although in English it has roughly the same meaning - for example as an answer to the question "Have you been sleeping well?" "If you don't count last night." or "If last night doesn't count." Either could be used to answer the question, but "If you don't count last night." is the correct translation for the sentence as written.