Hi @Frans1959,
Could you please share a bit more about your scenario?
Further, could you please share more details about the update statement of Date and time picker that you mentioned?
I have made a test on my side, and don't have the issue that you mentioned. The screenshot as below:

Note: The DateTimeZone property of the Date picker control within my Edit form set to following:
DateTimeZone.Local
I think this issue is related to the time zone. The update statement of date and time pciker that you mentioned (as below):
DateAdd(SetTimeStart;-TimeZoneOffset(SetTimeStart);Minutes)
The above formula is used to convert a UTC time into your local time. If you want to use the above formula to convert time, you must make sure that the SetTimeStart variable stores a UTC time. I suppose that you have set the DateTimeZone property of the Date picker control to DateTimeZone.UTC, is it true?
More details about converting from UTC time within PowerApps, please check the following article:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/functions/function-dateadd-datediff#converting-from-utc
Please check if the DateTimeZone property of the Date picker control within your Edit form is set to following:
DateTimeZone.UTC
Please take a try to set the DateTimeZone property of the Date picker control within your Edit form to following:
DateTimeZone.Local
and then try your variable (SetTimeStart) again, check if the issue is solved.
Best regards,
Kris