Among other things, Hippiemancy is the magic of relationships: detecting and altering the forces of opposition and attraction which bind people or units. Date-a-mancy is the oddest branch of Hippiemancy, in that it follows the Numbers which underlie all action. Date-a-mancy tries to quantify the intangible, things like leadership and compatibility and morale and Loyalty and Duty and even love, by means of match-ups. This can be very useful in warfare and logistics, but tends to wreak havoc among personal and political relationships. Sides tend not to keep their Date-a-mancers very long, because knowing the cold truth behind our interpersonal relationships only causes grief.
- Date-a-mancy was used in the creation of the Laurel of Napster, alongside Hat Magic, in a Caster Link. Considering the Magic Item is worn on the head, we can assume that the greater part of the abilities of the item are Date-a-mantic in nature. The item allows its wearer to instantly and accurately surmise the likely result of any two units engaging in direct single combat. In this way a Warlord can choose the most advantageous pairings in a fight, and improving his side's chances of success. Therefore it is likely that one aspect of Date-a-mancy is the instantaneous processing, and possibly the intuitive acquisition, of important data. Contrasting this with Mathamancy as in Parson Gotti's Bracer, the Bracer provides a precise numerical probability of success or failure in a particular venture, whereas the Laurel simply provides a generalized advantage/disadvantage report (with some indication of severity). The Bracer would tell you how likely you were to win a combat overall, while the Laurel would tell you who would have an advantage if the two best fighters were to square off, for instance.
Magical disciplines on Erfworld often include a variety of different applications, each of which is based on a different interpretation of the discipline's name. For example, Turnamancy grants power over both the number of turns it takes to pop a unit, and the turning of prisoners (changing Loyalty).
1) The term "Date-a-mancy" suggests a particularly interesting potential combination of abilities. First, since 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. Date-a-mancy 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").
2) Perhaps the best evidence into the nature of Date-a-mancy is where it falls within, and thus its connotations with Hippiemancy and a date-a-mancy influenced Laurel owned by Duncan Scone, chief warlord of Faq. 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 stuff that already exists currently (no control over Time.)  Foolamancy (also of Life and Numbers like Date-a-Mancy) appears to manipulate the appearance (Numbers) of existing beings including making non-empty space appear empty (again no control over Time.) In this way, coupled with the apparently experiential nature of Hippiemancy, Date-a-Mancy (Matter, Life, and Numbers), may allow a Date-a-Mancer to perceive, create, or replicate meaningful experiential connections (Life) of events (Stuff) acausally  that is to say with control over Time or in other words with the ability to manipulate meaningful experiential connections across, between, or irrespective of 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" coincidence 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  or the experience of classically defined synchronicity .
Finally, the descriptions (above) now have evidence in the date-a-mancy influenced artifact owned by Duncan Scone, chief warlord of Faq. The magic Laurel allows Duncan to do match up scenarios of two units (or perhaps stacks) in a battle and see who is most likely the victor. In this way, he can pick optimal match-ups and not waste stronger units overpowering weaker (overkill) or the reverse despite unit appearances. This power in addition to predicting the future also has the pun of not being unlike a 'date' between two battling units.