@RoyaaIn the Fitbit app, go to account settings, then tap the gear icon at the top right. Scroll down to location settings and ensure that Automatic Time Zone is switched on, the resync. This should solve the problem if it was previously off. Mine has been off before after an update.
If you want your time zone to be fixed, switch that option off, and manually set your time zone in the option below Automatic Time Zone.
Hope that does it!