Characters frequently make reference to "days", and some places within the Mess do have a standard day/night cycle. Often, however, "days" are measured relative to the character's own sleeping cycle.
Upwarp runs on Upwarp Standard Time (UST), which is as near to absolute time as their engineers can manage. In practice, seconds and hours must often be added or subtracted from the time scale to account for discrepancies, but these are kept to a minimum by the heroic work of the Department of Time Stabilization.
Strange has its own arcane and incomprehensible timekeeping system - used most notably by the Strangewarp Library. Users are advised to return their books promptly to avoid late penalties.
Some cultures within Top and Bottom have a calendar based on natural rhythms, though the former is seasonal and the latter more to do with fertility cycles. Chitin Queen calendars often include aspects of multi-year brood cycles.
These Units of time are rather 'slippery' rather than discrete units, as these are stories we are telling and not war games nor chess moves. And it's vaguely pretentious -:)
On Measurement & Time: A Notion Measurement could be something that one being could touch. A Scene could be something that takes an effort to move to/use/etc. And a Wheel could be something that is so immense as to be pretty much beyond description.
Trilogee thought up one on her own.
I want to build a clock that ticks once a year. The century hand advances once every one hundred years, and the cuckoo comes out on the millennium. -- [Danny Hillis]
Time, like water, flows downhill, and carries a lot of crap with it. - Echo