we have an app in production , which has a remote repository on Gitlab,
sometimes when we locally git merge , old changes overwrites new changes(without git saying anything or warn us) , and when we release a new version , some old and previously resolved bugs show up again.
does a remote repository supposed to calibrate date/times of local's? or we should do it manually?
No. Git commit timestamps are not used by the merge logic. Whatever is causing your issue, it's not due to the timestamps on the commits.