The ftime() function, introduced in V7 Unix (1979), gets the current
time in seconds and milliseconds, and time zone information.  It was
marked as a legacy interface in POSIX.1-2001, and removed altogether
from POSIX.1-2008.  The gettimeofday() function, originally from
4.1 BSD, gets the current time in seconds and microseconds, and optional
time zone information, and was marked as obsolete in POSIX.1-2008
although it was kept in the standard.  The POSIX recommended function
for getting time with sub-second resolution is clock_gettime(), which
was introduced in POSIX.1b-1993 and is now part of the base POSIX
standard; it supports multiple clocks and nanosecond resolution.
Additionally the function timespec_get() was introduced in C11 and also
supports nanosecond resolution.

To support dates beyond the year 2038, glibc and other libraries are
being updated to support 64-bit time_t even on 32-bit architectures,
requiring new implementations of interfaces that work with time.  As
part of this effort, the ftime() function was deprecated in glibc 2.31
(released February 1, 2020), a warning is now issued when building code
that uses this function, and removal is planned for a future version of
glibc (https://sourceware.org/pipermail/libc-announce/2020/000025.html).

ftime() is used in http.c to measure time intervals with millisecond
resolution.  To avoid the glibc 2.31 deprecation warning and further
issues when the function is removed entirely from glibc, clock_gettime()
is now used instead when it is available in the C library, as it is on
current Linux systems.  Prior to glibc 2.17, clock_gettime() required
linking with librt; on such systems ftime() will continue to be used, to
avoid an additional library dependency.  macOS provides clock_gettime()
starting in macOS 10.12; earlier versions will continue to use ftime().
Windows provides ftime() but not clock_gettime(), so ftime() will
continue to be used on Windows.

ftime(), gettimeofday(), and clock_gettime() with the CLOCK_REALTIME
clock get the "real time", which is subject to jumps if set by an
administrator or time service.  The CLOCK_MONOTONIC clock does not have
this problem and is more suitable for measuring time intervals.  On
Linux, the CLOCK_BOOTTIME clock measures the time since last boot and is
the same as CLOCK_MONOTONIC except that the Linux CLOCK_MONOTONIC clock
does not advance when the system is suspended.  Because it is used to
measure time intervals, CLOCK_BOOTTIME or CLOCK_MONOTONIC are used when
available, when clock_gettime() is used.  However the only clock
required by POSIX.1-2008 is CLOCK_REALTIME, so that will be used if the
other clocks are not available.