So I tried using it like:

engine.addOperatorDecorator('dateValue', (factValue, value, next) => next(factValue, new Date(value).getTime()));

And I'm setting a fact like:
currentDate = new Date().getTime();

If you log that, you get the ms since Unix epoch:
currentDate 1738883049146

And then a rule like:

{
 "all": [
 {
 "fact": "currentDate",
 "path": "",
 "value": "2025-02-05",
 "operator": "dateValue:greaterThan"
 }
 ]
}

And it works as expected with:
'lessThan', 'greaterThan'

But for most practical purposes, 'equal', 'notEqual', 'lessThanInclusive', 'greaterThanInclusive' are not going to be useful then, as you're essentially never going to be comparing exactly that ms of time. I only want to look at the actual days involved, not the time. And I want the same results whether it's 5am or 10pm local time - not based on UTC. If it's being used to set something based on the date, it needs to be the date where I live.

So I've setup a function like this (yeah, vanilla JavaScript sucks with handling dates, and I could shorten this, but like this for clarity):

function getStartOfCentralTimeDayInMs(dateOnlyString) {
 let localDateTimeString;
 if(dateOnlyString){
 // console.log("dateOnlyString",dateOnlyString);
 const localDate = new Date(dateOnlyString+'T00:00:00.000Z'); // Fake UTC for comparison
 localDateTimeString = localDate.toLocaleString("en-US", {
 timeZone: "UTC",
 });
 // console.log("localDateTimeString fact",localDateTimeString);
 } else { // Get the date and time locally / current
 localDateTimeString = new Date().toLocaleString("en-US", {
 timeZone: "America/Chicago",
 });
 // console.log("localDateTimeString current",localDateTimeString);
 }
 // Create a new Date object from the local time string
 const localDate = new Date(localDateTimeString);
 // Set the time to the beginning of the day (00:00:00.000)
 localDate.setHours(0, 0, 0, 0);
 // Get the date in milliseconds
 const startOfLocalDayInMs = localDate.getTime();
 return startOfLocalDayInMs;
}

And then set the fact as this:

currentDate: getStartOfCentralTimeDayInMs(null)

And then setup the addOperatorDecorator like this:

engine.addOperatorDecorator('dateValue', (factValue, value, next) => {
 const ruleValue = getStartOfCentralTimeDayInMs(value);
 return next(factValue, ruleValue);
});

(not a one-liner so that I can log values within the function for testing)

And the rule can be the same as above.

Then my date numeric values are for the start of the day in the local timezone, and look something like: 1738908000000

This lets me use any of these for date logic:
'lessThan', 'greaterThan', 'equal', 'notEqual', 'lessThanInclusive', 'greaterThanInclusive'

Hopefully this helps someone else.

@chris-pardy Thanks for your help on this - would it be worthwhile adding this to the documentation for handling dates?