A day and a time held together as one value. Precision enough to order events that share a date.
English compound of date (Latin data, given) + time (Old English tima, a portion of duration); a day and a time held together as one value, precise enough to order events that would otherwise share a single date.
Its own two halves come from different centuries of English, simply glued end to end, a given-day joined to a portion-of-duration with nothing translated between them. The join exists only so two events on the same date can still be told apart.