I doubt Calendar Man will ever appear in the show, since a character loosely based on him (Calendar Girl) was created for the series.
POSSIBLE VOICE ACTOR CHOICE: Matt Frewer
Known for his performance as Max Headroom, Mr. Frewer has appeared as Sid the Squid in a prior episode. He has the perfect manic personality for the weird Calendar Man
CALENDAR MAN'S COMIC ORIGIN
REAL NAME: Julian Day
FIRST APPEARANCE: Detective Comics # 259
ORIGIN: In the guise of Maharajah the Magician, Julian Day arrived in Gotham City for a five day engagement, which was actually a front for the series of crimes he intended to commit as the Calendar Man. Basing his crimes on the five different seasons (the fifth being the Indian monsoon or rainy season) -Calendar Man wore a different uniquely equipped uniform for each crime, publicly challenging the Batman to catch him before he could finish his crime spree. Unfortunately for Day, Batman was more than equal to the task. Day has returned several times, always basing his crimes on calendar-related themes. He recently appeared (in Julian Day mode) in the excellent 'The Long Halloween' by Loeb and Sale.
((from 'Who's Who in the DC