For Casper blockchain to maintain the Chain's trust at a top-notch level, Casper designates timestamps for every unique validator.
Casper measures time in milliseconds since the epoch7. An integer timestamp is called a tick. Without loss of generality, Casper assumes that the participants have synchronized clocks.
Casper pseudorandomly assigns one of the validators to each tick as the tick’s leader. One needs to make sure that this schedule is sufficiently random so that honest validators appear in this schedule frequently.