You did not get an answer to that indirect question on how to merge a separate date and time Memento fields into a combined datetime field. Actually you will find that date fields often will have a time setting as well, just ignored. The same with a time field.
But to merge those two fields:
ver e as entry();
var ddate = new Date(e.field("datefield"));
ver dtime = new Date(e.field("timefield"));
ddate.setHours(dtime.getHours(), dtime.getMinutes(), dtime.getSseconds(), dtime.getMilliseconds());
e.set("datetimefield", ddate.getTime());