Really it's only one level above the prisoner counting the days by scratching tally marks on the cell wall.
But they used to have them in offices and stuff long after actual computers and stuff existed. Now you've made me think about it that is actually really weird. The only thing I can think is that an automatic calendar is a lot harder to programme than a clock cos each month has a different number of days and the number of days in a year can also vary. It's not a repeating thing in the same way, you need to set it to the unique actual date if you want it to be right forever - does that make sense?