Review the cases when It's is used:
To describe the weather: It's sunny today, isn't it?
To identify the season: It's winter.
To identify the time of day: It's eight o'clock. It's late. It's morning.
To express an opinion: It's interesting. It's boring. It's funny. It's useless.
To make a statement: It's time to finish your work. It's time to go home.