Late last year Delta announced some pretty major changes to the SkyMiles program, including the introduction of actually useful upgrade instruments for top tier elites. While the number of upgrade instruments earned decreases, their value greatly increases, as Diamond Medallion members will have the option of receiving four systemwide upgrades without fare restrictions.
Anyway, something I apparently missed in the announcement is that the Medallion status year also changed. Per the SkyMiles updates page on delta.com:
The Medallion year traditionally begins March 1 each year to allow time for Medallion membership packages to be delivered. With the streamlined production of membership packages and the availability of digital SkyMiles and Medallion cards in the Fly Delta app, we are able to begin the Medallion year earlier. Starting in 2016, the Medallion year will begin February 1 and subsequently, the 2015 Medallion year will end on January 31, 2016.
So ultimately status will be valid for one less month. Delta is basically giving two years advance notice here so we can’t really blame them (unlike their SkyMiles award chart changes with no advance notice).
(Tip of the hat to Mac)