- As of yet Unknown.
As of yet, no known Date-a-mancer has been introduced.
As this magic is aligned with Numbers and with Hippiemancy, "date" may have a meaning similar to a calendrical date, making date-a-mancy an analog of numerology or astrology in some form or fashion. Another option playing on this relationship is interpreting the name as its homophone "datamancy" - the processing of information, like Mathamancy but with less numeric sorts of information.
However, it may also refer to "date" in the sense of a romantic pairing, i.e. "going on a date", and therefore be involved with love spells. This would be consistent with Hippiemancy ("Free Love").
Perhaps the best evidence into the nature of Date-a-mancy is where it falls within, and thus its connotations with Hippiemancy. With their confluence of Life and Matter and their absence of Motion Classes, it may be that within Hippiemancy Flower Power (Erf) might influence the environment/nature (space), Signamancy (Fate) might influence perspective/insight (vision), and Date-a-mancy (Numbers) might influence synchronicity . Synchronicity may be best understood as "time in meaningful events", or put more clearly from the typical perspective, "the inverse of meaningful events in time".
In this way, Date-a-Mancy may detect or impact meaningful events across dates and time. For example, Dittomancy (also of Matter and Numbers like Date-a-Mancy) appears to create causal replicas (Numbers) of what already exists currently (no control over Time.)  Foolamancy (also of Life and Numbers like Date-a-Mancy) appears to create causal imaginary replicas (Numbers) of what imaginably exists (again no control over Time.) In this way, coupled with the apparently experiential nature of Hippiemancy, Date-a-Mancy (Matter, Life', and Numbers), may either, perceive, create or replicate meaningful experiential connections acasually  (with control over Time), that is across or between different dates and times.
So what's the practical meaning of this? It could manifest in multitude of ways, but here are some examples 1) if Date-a-Mancy can give perception it might manifest as a Date-a-Mancer having the facility for accurate assumptions, as every wild guess made turns out to be true, 2), if Date-a-Mancy can create it might manifest as a Date-a-Mancer having the ability to cause "the most unpredictable yet possible" outcome to occur which would be likely unknowable in advance to the Date-a-Mancer or anyone else, yet be literally reasonably explained post event, 3) if a Date-a-Mancer can replicate it might manifest as the experience of Déjà vu .