Hi @linky_07,
This is because flow currently only supports UTC+00. We are planning to add support for Timezones in flow.
A workaround for now is to change the timezone from the sharepoint list to UTC+00.
You can also try to use the Workflow Definition Language addhours in compose action to change to the correct time:
1. First add the dynamic content (For example, the [Start Time] here) into the formula, just like you did before:
"@Addhours(body([Start Time], -1)"
2. Then save the flow, it will show you that the formula is not right, which would expose the code used for the dynamic content in the error messages part:

3. After that, remove the @{} symbol, which would leave only the contents body('Create_event')['Start'], then use this as the function parameter.
Regards,
Mona