Sometimes those differences of many days are explainable by confusion of days with months, for example, 2/10 can be the second of October or the tenth of February, which are a long way apart.
Try checking several different timezone settings:
see Admin / diagnostics / phpinfo to check your effective PHP timezone, and your php.ini path, in case you need to change it
the system date format in Admin / Locale
the relevant user profile sections in each user’s profile.
I think I remember seeing an issue on Github that also had differences between what happens when you log in as normal user, versus as an admin. I can’t find it now, but if you want to look it might have some clues…
I checked the timezone in the diagnostic information. The default timezone is “Europe/Amsterdam” which should be correct (local and master value). The php.ini path seems to be file (/usr/local/lib/php.ini).
The Admin/ Locale settings also seem to be correct. The notation also corresponds to the locale used in Windows on the computer.
The user profiles also seem to be correct. Likewise, the notation corresponds to the locale/ notation used in Windows on the computer.
In addition we found out that strangely enough it has no consequences for planned items. It seems solely a presentation issue.