Time traits requirements

In the table below, X denotes a time traits class for time type Time, t, t1, and t2 denote values of type Time, and d denotes a value of type X::duration_type.

Table 31. TimeTraits requirements

expression	return type	assertion/note pre/post-condition
`X::time_type`	`Time`	Represents an absolute time. Must support default construction, and meet the requirements for `CopyConstructible` and `Assignable`.
`X::duration_type`		Represents the difference between two absolute times. Must support default construction, and meet the requirements for `CopyConstructible` and `Assignable`. A duration can be positive, negative, or zero.
`X::now();`	`time_type`	Returns the current time.
`X::add(t, d);`	`time_type`	Returns a new absolute time resulting from adding the duration `d` to the absolute time `t`.
`X::subtract(t1, t2);`	`duration_type`	Returns the duration resulting from subtracting `t2` from `t1`.
`X::less_than(t1, t2);`	`bool`	Returns whether `t1` is to be treated as less than `t2`.
`X::to_posix_duration(d);`	`date_time::time_duration_type`	Returns the `date_time::time_duration_type` value that most closely represents the duration `d`.