Telling the time requires a complex set of understandings; while it isn't something that preschoolers will pick up easily, you can begin to sow the foundation by learning the language of time and looking at clocks. Time is an abstract concept, that we have a precise name for every minute, every second even, that has fallen in history is a wild concept to grasp.
Start talking about time during the day, note that "It's eight o'clock, let's have some breakfast", "It's ten o'clock, let's have a snack", "It's half past twelve, let's have some lunch", and so on. Young children will begin to understand that there are names for points of the day, and that certain things happen at those points. If you have a chiming clock at home or you live within earshot of a town or church clock, then start to observe the 'bongoing' and have your little ones tell you what hour it is.
Look at a toy clock face and look at where the hands are pointing. Again, even a 'hand' 'pointing' to a number is a concept that has to be understood. Move the hands and show how they point to different numbers, this will help your little ones understand what you mean. Of course, they also need to be able to recognise their numbers up to twelve, and again, a clock face is great for counting and learning these numbers.
Familiarity at a young age will help your children to learn the rest of time telling more easily in due course. They will probably only really grasp this when they reach six or seven, but introduce them to clocks and the language of time and they will begin to understand the whole process.