Julian Gregory Day was a crook fascinated with dates, and used his extensive genius to adopt various aliases as the Calendar Man modeled after a seasons and times of the year he was at that point obsessed with, having limited success. However, as he operated in Gotham City on Earth-One, he was bound to come into conflict with the Dynamic Duo of Batman and Robin. When a haphazard clue led the pair to his latest identity, he was unprepared and met his defeat.
He wasn't, however, the first "Calendar Man" to appear in comics... nor in one of Batman's series. A golden age version, actually originating from the year 2043, began a crime spree in 1943. Rob Callender was a laboratory assistant longing for the bygone days on Earth-Two when Batman was among the premier crime fighters. The thrill of becoming a super-criminal and gaining fame along with fortune consumed him, and as it turned out a laboratory accident propelled him through a time warp into his future...er...past foe's day. Using his extensive knowledge of scientific gadgetry from his time, Callender recruited a gang and began accumulating historical treasures instead of money. However, the time warp pulled him back at the end of his capers, leaving behind his seeming treasure which Batman claimed for himself when its owners didn't want it returned!
However, Calendar Man would returned years later and become a recurring adversary of the silver age Batman, continuing to changing his appearance, equipment and tactics modeled after his affection for the days and seasons. Only he never quite learned, his time was up!