Add time type conversion info to docs

doc/src/usage.rst

@@ -633,6 +633,28 @@ to map `date.max` to :sql:`infinity`, for instance::
 Of course it will not be possible to write the value of `date.max` in the
 database anymore: :sql:`infinity` will be stored instead.
 
+.. _time-handling:
+
+Time handling
+'''''''''''''
+
+The PostgreSQL :sql:`time` and Python `~datetime.time` types are not
+fully bidirectional.
+
+Within PostgreSQL, the :sql:`time` type's maximum value of `24:00:00` is
+treated as 24-hours later than the minimum value of `00:00:00`.
+
+    >>> cur.execute("SELECT '24:00:00'::time - '00:00:00'::time")
+    >>> cur.fetchone()[0]
+    datetime.timedelta(days=1)
+
+However, Python's `~datetime.time` only supports times until `23:59:59`.
+Retrieving a value of `24:00:00` results in a `~datetime.time` of `00:00:00`.
+
+    >>> cur.execute("SELECT '24:00:00'::time, '00:00:00'::time")
+    >>> cur.fetchone()
+    (datetime.time(0, 0), datetime.time(0, 0))
+
 
 .. _adapt-list: