When you get so engrossed in something which can either be a job in hand or the thing which you love to do most, you lose track of time.
Or in other words you don’t feel the passing of time and you hardly care about it’s existence when you are lost in your current job. You exist in the ‘Now’.
This is living in the present when you don’t worry about the future and don’t regret over your past. This is how a life should be lived.
A happy man is too satisfied with the present to dwell too much on the future – Albert Einstein