In a previous post I considered the weird initials added (in the 19th century, I believe) to Wellesley MS 29, a manuscript that has previously been dated "s. XVex/ XVIin", or "c. 1500", presumably partly on stylistic grounds, and partly due to a calendrical diagram on fol. 13r, which has the year "1500" written above it, using the medieval forms of arabic numerals (shown above).
Such diagrams are not uncommon, often appearing in pairs: one for finding the Dominical Letter [Wikipedia] and whether or not it is a Leap Year; the other for finding the Golden Number [Wikipedia]. They both appear in the Wellesley manuscript, underneath the "1500":
|Wellesley College, MS 29 [Source]|
Wikipedia], which forms the outer circle of the diagram); and "c." (among the Dominical Letters, which form the inner circle):
The only years from 1480 to 1515 with the Dominical Letter C were 1490, 1501 and 1507.
Written around the cross is "Hic dic unu(m)"; next to the number and letter for the following year is "duo", and next to the one after is "tria, etc." This seems to indicate the way the years can be counted 1501, 1502, 1503, etc., by adding 1, 2, 3, etc. to the "1500" written above. Thus, it seems to me, this diagram, and presumably the rest of the manuscript, was written in 1501, the year indicated by the cross on the diagram.