A calendar is a system for organizing days into larger units. Because Ishtar as neither moon nor pronounced seasons, the calendar systems are more esoteric. The two major "long period" objects used are
Lucifer and
Icaraus.
The Long Count is based on the synodic period of Lucifer and Ishtar, which align every 13 anna or 8 terran years. It is an ancient system, which was created before any extant civilization on Ishtar.