Add explicit time zone to datetime exported to ical.

* org-icalendar.el (org-icalendar-use-UTC-date-time): remove.
(org-icalendar-date-time-format): New custom variable.
(org-icalendar-use-UTC-date-timep): New function.
(org-ical-ts-to-string): Use the new variable.

----
When exporting to ical, using localtime is incompatible with some
software, using explicit universal time may failed with daylight
saving time, so we need another possibility, that is localtime with
explicit timezone.

lisp/org-icalendar.el

@@ -199,12 +199,25 @@ When nil of the empty string, use the abbreviation retrieved from Emacs."
 	  (const :tag "Unspecified" nil)
 	  (string :tag "Time zone")))
 
-(defcustom org-icalendar-use-UTC-date-time ()
-  "Non-nil force the use of the universal time for iCalendar DATE-TIME.
-The iCalendar DATE-TIME can be expressed with local time or universal Time,
-universal time could be more compatible with some external tools."
+(defcustom org-icalendar-date-time-format ":%Y%m%dT%H%M%S"
+  "format-string for exporting icalendar DATE-TIME.
+See `format-time-string' for a full documentation. The only
+difference is that `org-icalendar-timezone' is used for %Z
+
+Interesting value are:
+ - \":%Y%m%dT%H%M%S\" for local time
+ - \";TZID=%Z:%Y%m%dT%H%M%S\" for local time with explicit timezone
+ - \":%Y%m%dT%H%M%SZ\" for time expressed in Universal Time"
+
   :group 'org-export-icalendar
-  :type 'boolean)
+  :type '(choice
+	  (const :tag "Local time" ":%Y%m%dT%H%M%S")
+	  (const :tag "Explicit local time" ";TZID=%Z:%Y%m%dT%H%M%S")
+	  (const :tag "Universal time" ":%Y%m%dT%H%M%SZ")
+	  (string :tag "Explicit format")))
+
+(defun org-icalendar-use-UTC-date-timep () 
+  (char-equal (elt org-icalendar-date-time-format (1- (length org-icalendar-date-time-format))) ?Z))
 
 ;;; iCalendar export
 
@@ -652,12 +665,11 @@ a time), or the day by one (if it does not contain a time)."
 		(setq h (+ 2 h)))
 	    (setq d (1+ d))))
 	(setq time (encode-time s mi h d m y)))
-      (setq fmt (if have-time (if org-icalendar-use-UTC-date-time 
-				  ":%Y%m%dT%H%M%SZ"
-				  ":%Y%m%dT%H%M%S")
+      (setq fmt (if have-time 
+		    (replace-regexp-in-string "%Z" org-icalendar-timezone org-icalendar-date-time-format)
 		    ";VALUE=DATE:%Y%m%d"))
       (concat keyword (format-time-string fmt time 
-					  (and org-icalendar-use-UTC-date-time 
+					  (and (org-icalendar-use-UTC-date-timep)
 					       have-time))))))
 
 (provide 'org-icalendar)