I work for a municipality too mate, feel your pain. tried responding to your thread but posted to the main one.
Only way I've found around the problem of AGOL not accepting anything but UTC from your non-hosted services is to:
- go to the pop up configuration for the layer in your web map
- Add Attribute expression
- for the expression, pick the function ToLocal in the functions tab, and in the brackets put your time field from the globals tab. eg my time field is Last_Position, so the expression is ToLocal($feature["Last_Position"])
- Edit the name of the expression if you like to something other than Custom
- once you click OK, you'll have access to this new attribute in the fields available to you. configure your pop-up to show this field and you'll get your time converted to your local timezone.
I'll repeat my original plea for AGOL to accept the Data Field Setting in the map services.