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### 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`.