bpo-29026: Clarity documentation of time.time

Clarify the documentation of time.time by more
precisely defining what is meant by "seconds since
the epoch" on most platforms. Additionally explain
how gmtime and localtime may be used to extract
calendar components and convert to a more common
date format.

Doc/library/time.rst

@@ -19,9 +19,16 @@ An explanation of some terminology and conventions is in order.
 
 .. index:: single: epoch
 
-* The :dfn:`epoch` is the point where the time starts.  On January 1st of that
-  year, at 0 hours, the "time since the epoch" is zero.  For Unix, the epoch is
-  1970.  To find out what the epoch is, look at ``gmtime(0)``.
+* The :dfn:`epoch` is the point where the time starts, and is platform
+  dependent.  For Unix, the epoch is January 1, 1970, 00:00:00 (UTC).
+  To find out what the epoch is on a given platform, look at ``gmtime(0)``.
+
+.. index:: seconds since the epoch
+
+* The term :dfn:`seconds since the epoch` refers to the total number
+  of elapsed seconds since the epoch, typically excluding
+  `leap seconds <https://en.wikipedia.org/wiki/Leap_second>`_.
+  Leap seconds are excluded from this total on all POSIX-compliant platforms.
 
 .. index:: single: Year 2038
 
@@ -572,12 +579,27 @@ The module defines the following functions and data items:
 
 .. function:: time()
 
-   Return the time in seconds since the epoch as a floating point number.
+   Return the time in seconds since the epoch as a floating point
+   number. The specific date of the epoch and the handling of
+   `leap seconds <https://en.wikipedia.org/wiki/Leap_second>`_
+   is platform dependent.
+   On Windows and most Unix systems, the epoch is January 1, 1970,
+   00:00:00 (UTC) and leap seconds are not counted towards the time
+   in seconds since the epoch. This is commonly referred to as
+   `Unix time <https://en.wikipedia.org/wiki/Unix_time>`_.
+
    Note that even though the time is always returned as a floating point
    number, not all systems provide time with a better precision than 1 second.
    While this function normally returns non-decreasing values, it can return a
-   lower value than a previous call if the system clock has been set back between
-   the two calls.
+   lower value than a previous call if the system clock has been set back
+   between the two calls.
+
+   The number returned by :func:`time` may be converted into a more common
+   time format (i.e. year, month, day, hour, etc...) in UTC by passing it to
+   :func:`gmtime` function or in local time by passing it to the
+   :func:`localtime` function. In both cases a :class:`struct_time` object
+   is returned, from which the components of the calendar date may be accessed
+   as attributes.
 
 .. data:: timezone