The timestamps you’re seeing are in UTC, not GMT. Most of the time they’re interchangeable, but GMT fluctuates with winter/summer time changes, while UTC does not.
If you set your formula to deduct 2 hours, it’ll work for the next couple of months but then be off by an hour when you switch back to winter time.
I really suck at templating, but have a read here. There should be the info you need to handle UTC > local time conversions properly.