Q: How do we measure time?
A: Using a clock.

Q: How do we know when an hour has passed?
A: We look at the starting position of the clock hand and the ending position. An event has occoured.

If an event has occoured we percieve it as a passing of time.

Of course, there are many events, so we have alot of measures of time.

But, this theory makes sense, as

When we are engaged in a fun activity, we may perhaps notice more events occouring, and hence time flies.
Likewise, when we are bored, and there are not many events occouring, time seems to pass slower.