Convert from DateTime item in JavaScript

I need to set the timeout on a timer that takes milliseconds.
I have a DateTime item.
I need to convert that to epoch milliseconds, then subtract from epoch now, and set the timer with that time delta.
This is JavaScript ECMAv11 using the rules editor.

I am unable to find documentation that describes how to get the epoch from a DateTime item.

You don’t need to do all of that. There’s a millisFromNow() method on ZonedDateTime and time,toZDT() will convert the DateTime Item or the rawState to a ZonedDateTime.

var delay = time.toZDT(items.getItem("MyDT")).millisFromNow();

or

var delay  = time.toZDT(items.getItem("MyDT").rawState).millisFromNow();

However, in all cases, as documented, an Item’s .state always returns the String representation of the state. And also as documented, the library can’t process the toString if it includes the timezone.