A quarter date is a date like “Q1 1978”. It means the first 3 months of 1978. “Q2 1978” is the next 3 months (April to June 1978) and so on. A quarter date is really a special kind of range (see the Range tab). Use a quarter date when you don’t know exactly when a date occurred, but you do know the quarter in which it occurred.
With fact dates (dates of events or attributes, displayed in the Facts tab of the Property Box), you can optionally also enter a separate sort date.