Time is a convention made by man to standardize made up moments in a 'time contineum'. (Example: A second is a made up unit, a minute is a made up unit) It would be monitored in the mind because it's how we perceive this made up notion, yet there is always a clock out there that will tell you exactly what time it is so you stay on track with the rest of the world.

None the less, to stay in the present you have to really put yourself into activities and focus on them. Daydreaming leads you to think of the future. Doing nothing and ruminating leads you to think of the past. It's normal to think of the future and past, but the challenge is pulling strength together to get what is needed now. However I would not discount the past or future. If you don't think about either at all then you'll never learn from what has happened and you'll never plan to be ahead.