[gnome-calendar] gcal-edit-dialog: Use event timezone if available

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit ac879dd8ecd46d2aa6ed37eb07fe49da47c85057
Author: Elias Entrup <elias-git flump de>
Date:   Sat Jun 2 11:36:30 2018 +0200

    gcal-edit-dialog: Use event timezone if available
    
    Changing the time of an event with a different
    timezone was automatically changing the timezone
    to the local one.
    This patch checks whether a timezone is already
    set and uses that.
    
    Fixes #273

 src/gcal-edit-dialog.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
---
diff --git a/src/gcal-edit-dialog.c b/src/gcal-edit-dialog.c
index 182afde8..a2478dc0 100644
--- a/src/gcal-edit-dialog.c
+++ b/src/gcal-edit-dialog.c
@@ -1467,7 +1467,17 @@ return_datetime_for_widgets (GcalEditDialog   *dialog,
 
   /* Use UTC timezone for All Day events, otherwise use the event's timezone */
   all_day = gtk_toggle_button_get_active (GTK_TOGGLE_BUTTON (dialog->all_day_check));
-  tz = all_day ? g_time_zone_new_utc () : g_time_zone_new_local ();
+  if (all_day)
+    {
+      tz = g_time_zone_new_utc ();
+    }
+  else
+    {
+      if (gcal_event_get_timezone (dialog->event))
+        tz = g_time_zone_ref (gcal_event_get_timezone (dialog->event));
+      else
+        tz = g_time_zone_new_local ();
+    }
 
   date = gcal_date_selector_get_date (date_selector);
   time = gcal_time_selector_get_time (time_selector);