"Saatiniz kaç?" is more polite than "Saat kaç?" for face to face conversation. When you don't know the time, you can ask someone who walking on sidewalk or sitting somewhere "Affederesiniz, saatiniz kaç?" :) If you want to ask your friends live in different time zone, you should say "Orada saat kaç?" or "Çin'de/Kanada'da şu anda saat kaç?".

Saat kac : what's the time? / what time is it? (general for asking the time) Saatiniz kac : what's your time? (more specific for asking time might be used for asking to someone with different time zone)

Saat kaç : what time is it Saatiniz kaç: means what time is on your watch

Saat kaç(what time is it?) and saatiniz kaç(How many hours) both sentence same, but second sentence very gentle than first sentence.