### Wait traits requirements

The `basic_waitable_timer` template uses wait traits to allow programs to customize `wait` and `async_wait` behavior. [Note: Possible uses of wait traits include:
— To enable timers based on non-realtime clocks.
— Determining how quickly wallclock-based timers respond to system time changes.
— Correcting for errors or rounding timeouts to boundaries.
— Preventing duration overflow. That is, a program may set a timer's expiry `e` to be `Clock::max()` (meaning never reached) or `Clock::min()` (meaning always in the past). As a result, computing the duration until timer expiry as `e - Clock::now()` may cause overflow. —end note]

For a type `Clock` meeting the `Clock` requirements (C++Std [time.clock.req]), a type `X` meets the `WaitTraits` requirements if it satisfies the requirements listed below.

In the table below, `t` denotes a (possibly const) value of type `Clock::time_point`; and `d` denotes a (possibly const) value of type `Clock::duration`.

Table 35. WaitTraits requirements

expression | return type | assertion/note pre/post-condition
--- | --- | ---
`X::to_wait_duration(d)` | `Clock::duration` | Returns a `Clock::duration` value to be used in a `wait` or `async_wait` operation. [Note: The return value is typically representative of the duration `d`. —end note]
`X::to_wait_duration(t)` | `Clock::duration` | Returns a `Clock::duration` value to be used in a `wait` or `async_wait` operation. [Note: The return value is typically representative of the duration from `Clock::now()` until the time point `t`. —end note]