pxDateTimeisPastOrFuture is an OOTB function alias rule that can be used in When rules. I do not know a specific When rule offhand that uses it but you can see it referenced when selecting conditions in a When rule. You will see "[a datetime] is in the [past/future]" listed in the options. When you select it, you will then see the input parameters specified in the function alias rule.

Another example is function alias pxIsInListOfValuesInPageList in a when rule pzIsInOrNotInListOfValuesComparator and here is screenshot of when rule:

Hope this helps!