Counting Years in imperial Rome

[Epictetus]

Here is an even better passage:

Constantine’s tricennalia fell on 25 July 335; see Burgess 1988:81-2 and Kienast 1990:296. Constantine unusually celebrated both the beginning and the end of his twentieth and thirtieth anniversary years, that is, in 325 and 326, and 335 and 336; see Burgess 1988:79 n. 6.

Burgess, Richard and Witakowski, Witold; Studies in Eusebian and post-Eusebian chronography, page 217.

This makes sense. Constantine apparently celebrated the anniversary of the first 365 days of his reign. Since he didn't come to power on 1 January his celebrations stretched over two calendar years.