Merge pull request #806 from nexusformat/805-time_zone

recommend providing time zone in NX_DATE_TIME

manual/source/datarules.rst

@@ -423,14 +423,22 @@ NeXus dates and times
 
 NeXus  :index:`dates and times <date and time>`
 should be stored using the `ISO 8601`_ [#]_  format,
-e.g. ``1996-07-31T21:15:22+0600``.
+e.g. ``1996-07-31T21:15:22+0600`` (which includes
+a time zone offset of ``+0600``).
+Note:  The time zone offset is always numeric or ``Z`` (which means UTC).
 The standard also allows for time intervals in fractional seconds
 with *1 or more digits of precision*.
 This avoids confusion, e.g. between U.S. and European conventions,
 and is appropriate for machine sorting.
-
-.. _ISO 8601: http://www.w3.org/TR/NOTE-datetime
-.. [#] ISO 8601: http://www.w3.org/TR/NOTE-datetime
+It is recommended to add an explicit time zone,
+otherwise the local time zone is assumed per ISO8601.
+The norm is that if there is no time zone, it is assumed
+local time, however, when a file moves from one country to
+another it is undefined. If the local time zone is written,
+the ambiguity is gone.
+
+.. _ISO 8601: https://www.w3.org/TR/NOTE-datetime
+.. [#] ISO 8601: https://www.w3.org/TR/NOTE-datetime
 
 
 .. compound::

nxdlTypes.xsd

@@ -509,14 +509,26 @@
 
 	<xs:simpleType name="NX_DATE_TIME">
 		<xs:annotation>
-			<xs:documentation>alias for the ISO8601 date/time stamp</xs:documentation>
+			<xs:documentation>
+			Alias for the ``ISO8601`` date/time stamp.
+			It is recommended to add an explicit time zone,
+			otherwise the local time zone is assumed per ISO8601.
+			</xs:documentation>
 		</xs:annotation>
 		<xs:restriction base="nxdl:ISO8601" />
 	</xs:simpleType>
 
 	<xs:simpleType name="ISO8601">
 		<xs:annotation>
-			<xs:documentation>ISO8601 date/time stamp</xs:documentation>
+			<xs:documentation>
+			ISO8601 date/time stamp.
+			It is recommended to add an explicit time zone,
+			otherwise the local time zone *is assumed* per ISO8601.
+			The norm is that if there is no time zone, it is assumed
+			local time, however, when a file moves from one country to
+			another it is undefined. If the local time zone is written,
+			the ambiguity is gone.
+			</xs:documentation>
 		</xs:annotation>
 		<xs:restriction base="xs:dateTime" />
 	</xs:simpleType>