It's time you simple present/simple past

[Volcano1985]

It's time you stop smoking

It's time you stopped smoking

What's the difference?

[engee30]

It's time you stop smoking

It's time you stopped smoking

What's the difference?

The difference is that you normally say the latter.

[Raymott]

It's time you stop smoking

It's time you stopped smoking

What's the difference?

Only the second sentence is correct.
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.

Other options:
It's time that you stopped smoking.
It's time for you to stop smoking.
The time has come for you to stop smoking.