REST services always return UTC datetimes. The idea behind that is that REST services are being called by other systems. If those systems decide to show the date to a user, they can convert it into that user’s timezone.
I suspect it will take the date/time (based on the time zone for this account) for the user you are calling the REST service with. You can control the time zone they are on by creating a “new web service user”. There is a button in the standard “Account_Overview” screen. This screen allows you to select the time zone for this user (it is essentially just the new account screen).
Worth giving this a try on a test project.