[m-rev.] for review: clarify the relationship between date_times and time zones

[Julien Fischer]

For review by anyone.

-------------------

Clarify the relationship between date_times and time zones.

library/calender.m:
    As above.

Julien.

diff --git a/library/calendar.m b/library/calendar.m
index 47f5a6866..b1b4665a9 100644
--- a/library/calendar.m
+++ b/library/calendar.m
@@ -36,6 +36,10 @@
 %---------------------------------------------------------------------------%

     % A point on the proleptic Gregorian calendar, to the nearest microsecond.
+    % A date_time carries no time zone information; it is the responsibility
+    % of code that creates and uses date_times to ensure that any two
+    % date_times passed to the same operation refer to the same time zone.
+    % To convert between local time and UTC, see local_time_offset/3.
     %
 :- type date_time.