No one is mentioning calendars, and that calendars haven't been consistent enough for 1200 years to make the claim being made in the article. Here are a couple of articles I found that show that the claim of knowing that this year's cherry blossom is "one day earlier" than one in the 1400s is pretty difficult, if not impossible, to substantiate.
https://web.archive.org/web/20070402230356/http://www2.gol.c...
https://en.wikipedia.org/wiki/Japanese_calendar