The question seems simple but is actually important and can change the course of a scenario.

How do we remove the time counters ?

Case - 1 All and then I apply any associated time effects.
Case - 2 I removed one by one and I apply the associated effects (if necessary) between each removed counters

In this case below , it changes many things , including the difficulty of a scenario:

We are in a 4-player game and there is only one time counter on the enemy. Treachery comes out, I have to remove 4 counters.

Case - 1 I remove the single time counter on the enemy. I finished the effect of treachery card. I make the forced time effect of the enemy, I discard an ally and I put two time counters on the enemy. At the end of the refresh phase, I remove a time counter.
=> One ally discarded in turn by the time effect of the enemy.

Case - 2 I remove one time counter. I make the forced time effect of the enemy, I discard an ally and I put two time counters on the enemy. I remover a time counter and then another. I make the forced time effect of the enemy, I discard an ally and I put two time counters on the enemy. I remove one time counter. I finished the effect of treachery card. At the end of the refresh phase, I remove a time counter. I make the forced time effect of the enemy, I discard an ally and I put two time counters on the enemy.
=> Three allies discarded in turn by the time effect of the enemy.

How time counters are removed completely changes the course of a party and especially its difficulty. This is not necessarily the only scenario that cycle where it can be the case.

Personally, I have always made the case - 2.

French community are divided on this subject. Someone have an official answer on that ?

The treachery says "remove Time counters" which suggests you that all the Time counters are removed once. Since the Time counters of "put 2 Time counters on" didn't exist at that moment, my bet is that 1.

I would agree. Because the effect says to remove a number of tokens, you remove as many as you can, all at once. If you only have 1 available, then you just remove the 1.

I agree with both users. Effects such as that one remove tokens all at once.