Feb 02, 2017 - 02:58 PM
In the case of indicating a fixed time, you use "il est". You never say "c'est 3 heures" to mean "it's 3 o'clock". However you use "c'est" to indicate a time duration, for example "c'est 3 heures pour aller a Paris" (it takes 3 hours to go to Paris). In this case you have to use "il est" because it's a fixed time. If you were to say "c'est presque une heure", it would indicate a duration, like "c'est presque une heure de train" (it's about an hour in train).
For most other purposes, unless you are making a concrete reference to a masculine person or neutral thing in particular, then use "c'est". For example, "C'est joli", means that an object is nice in an abstract point of view, as opposed to "il est joli", meaning that this particular object is nice in a concrete point of view. It's one of those usages that come easier with practice and listening to others.