I am using ver.1.46. Here the mission-editor uses a calender of 1985 (implied by the title) and faithfully writes the date you choose to the mission file. (e.g. March 1st is saved as month=3; day= 1;)
However when the mission is loading a calender of 1984 is used, this is bad news because 1984 was a leap year. This means that the above mission (with official mission date March 1st) will show up as February 29th on the loading screen when you are about to play the mission.
Which version are you using? It seems that the error has been reversed but is still there
(?)