Issue #2736: Documented how to compute seconds since epoch.
diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index de9ad44..c637f6a 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -721,6 +721,22 @@
It's common for this to be restricted to years in 1970 through 2038. See also
:meth:`fromtimestamp`.
+ On the POSIX compliant platforms, ``utcfromtimestamp(timestamp)``
+ is equivalent to the following expression::
+
+ datetime(1970, 1, 1) + timedelta(seconds=timestamp)
+
+ There is no method to obtain the timestamp from a :class:`datetime`
+ instance, but POSIX timestamp corresponding to a :class:`datetime`
+ instance ``dt`` can be easily calculated as follows. For a naive
+ ``dt``::
+
+ timestamp = (dt - datetime(1970, 1, 1)) / timedelta(seconds=1)
+
+ And for an aware ``dt``::
+
+ timestamp = (dt - datetime(1970, 1, 1, tzinfo=timezone.utc)) / timedelta(seconds=1)
+
.. classmethod:: datetime.fromordinal(ordinal)