One SLEEP is the period between the time when someone goes to sleep and the next time that same person goes to sleep. It is equivalent to one day on the outer world.

One CASSIAN is the amount of time it takes the floating continent, Cassia, to make one complete revolution around the sun. There are 56 sleeps in a Cassian.

One DEMICIRCLE is the amount of time it takes the floating continent, Tama, to make one complete revolution around the sun. There are 168 sleeps, or 3 Cassians in one demicircle.

One CIRCLE is the amount of time it takes the floating continent, Kjell, to make one revolution around the sun. There are 336 sleeps, 6 Cassians, or 2 demicircles in one circle.

The Milenians call the two demicircles in each circle, First and Second respectively. The three Cassians in each demicircle also have individual names: they are Halav, Protius, and Petra during the First demicircle, and Halav, Matera, and Petra during the Second demicircle (named after the principal Immortals of Milenia). The name of the demicircle is used to distinguish between the two Halavs and the two Petras, thus First Halav and Second Halav, and First Petra and Second Petra. At one time, there was also a First Protius and Second Protius, but as the cult of Matera became more prominent, Matera displaced Second Protius. Technically, the Cassians of Protius and Matera are called First Protius and Second Matera, but as the First and Second are already known, they are usually excluded when a Milenian gives the date. Sleeps and circles are numbered on the Milenian calendar. Thus a complete date can be formed: e.g. the 13^th sleep of First Petra in the 1114^th circle of the Second Empire (measured from the time the Milenians arrived in the Hollow World; although the Milenians have forgotten their outer world roots, they do know that they had a previous empire "somewhere else" that collapsed). The 1114^th circle corresponds with the year 1014 AC on the Thyatian calendar.

First Halav corresponds to Nuwmont/Vatermont

(First) Protius corresponds to Thaumont/Flaurmont

First Petra corresponds to Yarthmont/Klarmont

Second Halav corresponds to Felmont/Fyrmont

(Second) Matera corresponds to Ambyrmont/Sviftmont

Second Petra corresponds to Eirmont/Kaldmont