Demonstration of issue
To demonstrate this issue, let's take the following list of records from a SharePoint list of meetings. Notice the start and end time values.
From a Power App, let's now display these records in a datatable control. Notice how the start and end times don't correspond with the values that appear directly in SharePoint.
Why is there a mismatch between SharePoint and Power App time values?
SharePoint stores UTC datetime values. Power Apps uses the region of the browser to determine the time zone of the user. It uses this to calculate the offset when saving and displaying time values.
SharePoint, however, uses the settings in SharePoint to determine the time zone. Where there is a mismatch in time zones, this can account for ambiguities in time values.
By default, SharePoint sites apply a United States time zone and locale. Therefore, this issue often affects users in other regions. In this example, my computer settings were set to the UK region, and the SharePoint site was set to the default US region. This accounts for behaviour that's demonstrated.
How to set the SharePoint time zone
To set the timezone in SharePoint, click the cog icon and click 'Site information'. From the 'Site information' panel, click the 'View all site settings' link.
From the 'Site Administration' group, click 'Regional settings'.
From the regional settings page, we can now modify the time zone setting.
Once we correct the timezone setting, datetime values will appear more consistently between SharePoint and Power Apps.