Definition
The reference date describes the event in a formal, quantitative way. It is the date on which the underlying event is expected to occur, or the end point of an event which lasts a defined period. It serves to define the event more precisely and to avoid ambiguity.
The reference date is different from the announcement date, i.e. the point in time when a result is expected to become known. Both in turn are different from the trading cut-off date for settlement, the point in time when a result actually becomes known. Obviously, trades after cut-off are not "predictions" anymore and do not count.
Example
A prediction question for the annual U.S. inflation rate for 2017 will show a reference date of 31 December 2017, defining the year spanning from 1 January 2017 to 31 December 2017. Announcement date might be entered as 15 January 2018, given that the U.S. Department of Statistics will typically need two weeks to compile inflation data. If the Department actually announces the Inflation rate on 14 January 2018 at 12:07pm. This date and time must be entered as the trading cut-of date to avoid players gaming the system unfairly after the final result is already known.