Setting's a reasonably generic Medievalish Fantasy Universe (wherein I don't worldbuild in great depth, it's just sort of generic Location--but definitely not anywhere in Earth history)
Our Heroine needs to tell someone the approximate equivalent of "Look for the [logbook] from August 23d, [year that is 101 years ago]" without, ya know, anachronisms like months based on Roman gods and years based on Christianity. The year I can fudge by just saying "101 years ago" (though it sounds a tad off somewhow), but the month-and-day? Not so much. Especially since she's saying this in the neighborhood of 2-3 months in advance rather than, say, the present date or next week.
Specific date is flexible, but it needs to be near the end of summer/early fall, preferably on a date with some degree of astronomical significance.
(if it matters, the culture has a slightly Celtic feel to it)
edit: I'm thinking of going with something to the general effect of "Harvest Eve, 105 years ago--1063 on the priest's calender". Does that strike anyone as 1. anachronistic, 2. confusing, or 3. not something someone would say when specifying a date? I will likely not otherwise discuss dates/years.