docs: Move time related functions into single chapter.

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+# Time
+
+_NNG_ supports has support for time in the form of access to a
+system clock, and supporting timeouts for certain operations.
+
+## Time Type
+
+```c
+typedef uint64_t nng_time;
+```
+
+The {{i:`nng_time`}} type is used to represent a clock offset from a common base time,
+measured in milliseconds.
+
+The reference, or zero value, is some arbitrary point in time, most often sytem boot, but can
+be process start time or any other convenient reference.
+
+All threads within a process will use the same reference time, but be aware that different processes
+may use a different reference time.
+
+## Duration Type
+
+```c
+typedef int64_t nng_duration;
+
+#define NNG_DURATION_INFINITE (-1)
+#define NNG_DURATION_DEFAULT  (-2)
+#define NNG_DURATION_ZERO     (0)
+```
+
+The {{i:`nng_duration`}} time measures a time {{i:duration}} in milliseconds.
+Normally durations are positive, but some specific negative values are reserved.
+
+- {{i:`NNG_DURATION_INFINITE`}}: The duration essentially means forever.
+  This is most often used with a timeout to indicate that there is no timeout, the
+  operation should wait until it is complete, even forever.
+
+- {{i:`NNG_DURATION_DEFAULT`}}: This special value is used in some circumstances to
+  prevent overriding a default timeout. Some operations have a default maximum time,
+  and this value means that the previously established default should be used.
+  The precise meaning is context-specific.
+
+- {{i:`NNG_DURATION_ZERO`}}: A zero length duration is used to performan an immediate
+  poll.
+
+## Getting the Current Time
+
+```c
+nng_time nng_clock(void);
+```
+
+The {{i:`nng_clock`}}{{hi:clock}} function returns the number of elapsed
+milliseconds since some arbitrary time in the past.
+The resolution of the clock depends on the underlying timing facilities of the system.
+This function may be used for timing, but applications should not expect
+very fine-grained values.
+
+## Waiting for Duration
+
+```c
+void nng_msleep(nng_duration msec);
+```
+
+The {{i:`nng_msleep`}} function blocks the calling thread for at least the specified
+number of milliseconds.
+
+> [!TIP]
+> This function may block for longer than requested.
+> The actual wait time is determined by the capabilities of the
+> underlying system.
+
+## See Also
+
+[`nng_cv_until`][nng_cv_until],
+[`nng_sleep_aio`][nng_sleep_aio]
+
+[nng_cv_until]: ./synch.md#waiting-for-the-condition
+[nng_sleep_aio]: TODO.md