Your child at this point has cognitive abilities of thinking, reasoning and problem solving, allowing him to develop a sense of time. And while he may easily recognize day and night or autumn and spring, it might be difficult for him often to explain the actual time.
It’s fun when a 3 year old calls ‘night’ ‘day!’ But a 6 year old doing the same will not seem very amusing.
What you can do
You can start by getting a rather big clock on the playroom wall. Having an analog clock makes all the difference as they will have to see the clock and recognize the time. Help them read the numerals by first letting them learn the numbers on a clock on a piece of cardboard.
Also, read out the time every now and then and ask them the time on a regular basis, without letting them know that you are doing this to teach them. For teaching them minutes, make sure that they learn numbers till 60 and help them learn tables.