Thank’s a lot for your answer.
I installed the plugin and actually there is a time difference between the two. The server is one hour back.
What should I do ?
I’ve found a solution to my forms (not very proud, but it works) :
I’ve created a special template for the forms (to be able to unactive the language selector)
So I have 1 page for each forms, and I put only one language in all the languages version.