Time quantifies or measures the interval between events, or the duration of events. ~(wikipedia)~ The standard unit for time is the SI second. A second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. We use larger units: minute, hour, day, week, month, year, century. Those additional units make things hard.