the maximum time allowed before the policy expires. While the application can express this time in any units they desire, the class truncates to milliseconds.
typename DurationRep
a placeholder to match the Rep tparam for duration's type. The semantics of this template parameter are documented in std::chrono::duration<>. In brief, the underlying arithmetic type used to store the number of ticks. For our purposes it is simply a formal parameter.
typename DurationPeriod
a placeholder to match the Period tparam for duration's type. The semantics of this template parameter are documented in std::chrono::duration<>. In brief, the length of the tick in seconds, expressed as a std::ratio<>. For our purposes it is simply a formal parameter.