The following Years, Months, and Days are mentioned in the stories:
A moon calendar is also mentioned, especially in the latter stories. The priestesses of Skama, the moon goddess, use this system. The twelve moons are, in order:
Snow, Wolf, Seed, Witch, Ghost, Murderer, Thundar, Satyr, Harvest, Second Witch, Frost, and Lovers.
In the story, When the Sea King's Away, the use of decades is implied, or at the very least, the use of year cycles is stated. The story mentions an event that occurs on the seventh day of the seventh moon of the seventh year of the Sevens cycle.
In the Lankhmar, City of Adventure, game accessory published by TSR an entire calendar is presented including the elements above. TSR bases their version of the Lankhmar calendar on the Gregorian system, and invents many of the names of years, months, and days to fill in the blanks. The game accessory also includes a few holidays based on various story elements such as the rat plague and the Gods of Lankhmar. There is at least one inconsistency with their calendar and the stories. They list three intervening months between the Serpent and the Owl, whereas in the story Thieves' House, the author describes the last days of the Serpent leading into the beginning of the Owl.