A SlabDay is 105 seconds.
As there were originally no light/dark cycles imposed on SlabCitizens, the SlabDay was a time convention only. However, old habits die hard. Seacombe residents tend to conform to a SlabDay routine, MultipliCity being synchronised to the sun-strip dimming cycle, spins once every ten hours and the sun in AllWeather is currently traversing the interface on a twenty-hour loop. Even though The Strip and Smith are permanent night sections, they still display usage patterns that are influenced by the diurnal cycles of the day sections. So much for our tenCent culture.
See Time Decimalisation for further reference.