Is seven “なな” or “しち“?

I’ve been learning Japanese on Duolingo for some time, and when I first started to learn numbers (mainly to say how old someone was), it taught me that seven is なな　(“nana”). However, in a later unit when I learned more about telling time, it said that seven was しち　(“shichi”). For example, to say “my daughter is seven years old” it was “むすめはななさいです”. But to say “it’s seven o’clock” suddenly it became “しちじです” . Is there a reason for this?