Student or Learner
It's time you stop smoking
It's time you stopped smoking
What's the difference?
It's time you stopped smoking. (Subjunctive: the action is hypothetical; it's not happening.)
It's time you had a bath; It's time you left for work. Always the past tense - or rather the present subjunctive tense, which has the form of the past tense.
It's time that you stopped smoking.
It's time for you to stop smoking.
The time has come for you to stop smoking.