"il" can be a masculine pronoun or an impersonal one ("it"). When we say "il est 5 heures" it's like "it is 5 o'clock". it's impersonal. It's the same for "il fait 15 degrées" (it's 15C). It's not masculine nor feminine, it's just "it". Heure is feminine and "quel" conjugate with the word it belongs. So it can become "quelle" "quels" "quelles".

In my opinion, "il" is a indefinite pronoun. You can see it in some phrase, like "Il faut...". And "heure" is feminine so we use "quelle" here

Good question, SSema. I'd like to know the answer myself. This gender stuff is a bit confusing for me also.