Minerva has been teaching at Hogwarts for over a decade before Septima Vector arrives to take the Arithmancy post, and has more than a few years on her besides, but for those of a magical bent the years are less of a barrier than they are to muggles, especially for two who are well settled in their careers.
Septima is almost scarily hypercompetent at everything she does, switching from the new professor to the strict professor who accepts only the best for and from her students within the first week of classes, from the newest in the staffroom to capable college long before the first staff meeting, and from Minerva's acquaintance to companion precisely on her own schedule.
One of the reasons Minerva McGonagall, Deputy Headmistress, recommended hiring Septima was her tendency to hold to precision above all else; in her teaching – essential for destructive young magicals, given the potential product of mismanaged Arithmancy - in her work, and especially in her choice of partner, so once Minerva twigs to that fact she has no doubt a simple inquiry would net her the exact span of their first kiss, if she cared to ask.