Use coolreader or Duokan.
You will have a small clock displayed on to of the screen.
It may be possible to do something better with another hack, but there is none to my knowledge. (I new to this forum so it's not impossible).
Be careful, PW sometimes loses the clock (few hours more or less than the true clock).
It can be fixed by connecting on Wifi (careful of updates !) or by a command in the search bar : ;st (everything explained here :
Paperwhite not keeping time)
Hope it helps