What is time ?

What is time? the part of existence that is measured in minutes, days, years, etc., or this process considered as a whole:
spend time He wants to spend more time with his family.
time passes Time passes so quickly when you're having fun.
time goes on/by She grew more and more fascinated by the subject as time went on/by.
over/with time The curtains have faded over/with time (= as years have gone past).
in time You'll forget her in time (= in the future).
Over the course of time (= as years have gone past), holes have formed in the rock.
pass the time When Paula was ill, I took her some magazines to help her pass the time.
take more time with/over If you'd taken more time with/over (= spent more time doing) this essay, you could have done it much better.
take a time It takes a long time (= many hours are needed) to get from London to Sydney.
save time We'd save time on our journey (= it would be quicker) if we went by train.
period of time I only worked there for a short period of time.
gain/lose time The kitchen clock is gaining/losing time (= is going fast/slow).
keep time My watch has never kept very good time (= been correct).