HACKING/design: move 01c-time.md into doxygen.

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+/**
+
+@page time_periodic Time and periodic events in Tor
+
+### What time is it? ###
+
+We have several notions of the current time in Tor.
+
+The *wallclock time* is available from time(NULL) with
+second-granularity and tor_gettimeofday() with microsecond
+granularity.  It corresponds most closely to "the current time and date".
+
+The *monotonic time* is available with the set of monotime_\*
+functions declared in compat_time.h.  Unlike the wallclock time, it
+can only move forward. It does not necessarily correspond to a real
+world time, and it is not portable between systems.
+
+The *coarse monotonic time* is available from the set of
+monotime_coarse_\* functions in compat_time.h. It is the same as
+monotime_\* on some platforms. On others, it gives a monotonic timer
+with less precision, but which it's more efficient to access.
+
+### Cached views of time. ###
+
+On some systems (like Linux), many time functions use a VDSO to avoid
+the overhead of a system call.  But on other systems, gettimeofday()
+and time() can be costly enough that you wouldn't want to call them
+tens of thousands of times.  To get a recent, but not especially
+accurate, view of the current time, see approx_time() and
+tor_gettimeofday_cached().
+
+
+### Parsing and encoding time values ###
+
+Tor has functions to parse and format time in these formats:
+
+ - RFC1123 format. ("Fri, 29 Sep 2006 15:54:20 GMT").  For this,
+   use format_rfc1123_time() and parse_rfc1123_time.
+
+ - ISO8601 format. ("2006-10-29 10:57:20") For this, use
+   format_local_iso_time() and format_iso_time().  We also support the
+   variant format "2006-10-29T10:57:20" with format_iso_time_nospace(), and
+   "2006-10-29T10:57:20.123456" with format_iso_time_nospace_usec().
+
+ - HTTP format collections (preferably "Mon, 25 Jul 2016 04:01:11
+   GMT" or possibly "Wed Jun 30 21:49:08 1993" or even "25-Jul-16
+   04:01:11 GMT"). For this, use parse_http_time().  Don't generate anything
+   but the first format.
+
+Some of these functions use struct tm. You can use the standard
+tor_localtime_r() and tor_gmtime_r() to wrap these in a safe way. We
+also have a tor_timegm() function.
+
+### Scheduling events ###
+
+The main way to schedule a not-too-frequent periodic event with
+respect to the Tor mainloop is via the mechanism in periodic.c.
+There's a big table of periodic_events in mainloop.c, each of which gets
+invoked on its own schedule.  You should not expect more than about
+one second of accuracy with these timers.
+
+You can create an independent timer using libevent directly, or using
+the periodic_timer_new() function.  But you should avoid doing this
+for per-connection or per-circuit timers: Libevent's internal timer
+implementation uses a min-heap, and those tend to start scaling poorly
+once you have a few thousand entries.
+
+If you need to create a large number of fine-grained timers for some
+purpose, you should consider the mechanism in src/common/timers.c,
+which is optimized for the case where you have a large number of
+timers with not-too-long duration, many of which will be deleted
+before they actually expire. These timers should be reasonably
+accurate within a handful of milliseconds -- possibly better on some
+platforms.  (The timers.c module uses William Ahern's timeout.c
+implementation as its backend, which is based on a hierarchical timing
+wheel algorithm. It's cool stuff; check it out.)
+
+**/