The duration template uses the duration_values trait to construct special values of the durations representation (Rep). This is done because the representation might be a class type with behavior which requires some other implementation to return these special values. In that case, the author of that class type should specialize duration_values to return the indicated values.
Returns: Rep(0). [ Note: Rep(0) is specified instead of Rep() because Rep() may have some other meaning, such as an uninitialized value. — end note ]
Remark: The value returned shall be the additive identity.
Remark: The value returned shall compare less than or equal to zero().
Remark: The value returned shall compare greater than zero().