|Day length||30 h, 0 m, 43 s||30 h, 6 m, 9 s|
|Proteus "month"||1166 h, 47 m, 25 s||43 d, 26 h, 51 m, 50 s|
|Nereus "month"||1942 h, 06 m, 19 s||80 d, 03 h, 11 m, 2 s|
|Length of year||9979 h, 27 m, 14 s||331 d, 15 h, 31 m, 1 s|
(Siderial times are taken relative to the fixed stars; synodic (and solar) times are taken relative to the position of the sun overhead. Syndoic times are given in terms of local (solar) days.)
As the Poseidon year is a fraction over 331½ days, leap years occur more often than on Earth. The rule is:
- Odd-numbered years are leap years (332 days)
- Years that are multiples of 100 are leap years (332 days)
- Years that are multiples of 200 are double leap years (333 days)
Other, archaic, units of time are still used. The week is still defined to be seven days, and the month is defined as 30 days.
As for how Posiedoners spend their 30-hour days, another page describes the rhythm of a day on Poseidon.