Hi Marlowe!
An undead client may appear when you go to sleep for the night. By default, there is a 25% chance for an undead client to appear, but you can raise this to 100% by purchasing the Necromantic blessing from Atalia in The House of Blessings.
As it stands, there is no system tracking the hours of the day. The day ends when you go to sleep. Until then, you can do whatever your Stamina and Gold allows for as long as you like. It didn't seem prudent to include hours of the day, but if this is something players are interested in for the sake of realism, I might consider adding it in a future update.