Sure, I agree that GMT+2 is not the best option. However, it's apparently not obvious to all and sundry that
Date et heure de métropole refer to the date and time of metropolitan France. Perhaps it's obvious to you, but not to some (even seasoned) translators.
And one could use either
metropolitan France or
mainland France. However, if you really want to split hairs,
metropolitan France encompasses not only
mainland France but also Corsica and other islands in the Atlantic Ocean, the English Channel and the Mediterranean Sea. I also see
metropolitan France being used more often than
mainland France myself. See
https://en.wikipedia.org/wiki/Metropolitan_France