This is a post with respect to how combined date/time fields is handled in AX/Dynamics 365 for Operations (Dyn365FO). For more information on the type of date and time fields in AX visit the following link from docs.microsoft.com.
Dyn365FO stores combined date/time fields internally in the SQL database based on UTC time zone. Whenever the date is to be displayed to the user, an offset based on the user preferred time zone is applied and then shown to the user.
This preferred time zone is set in the following section in Dyn365FO.
System Administration-> Users -> Users
An excellent post that take a deeper dive on the subject along with the X++ code is found in the below link