Fix UTC time handeling, fixes #2195

Using the solution suggested by Sascha Silbe [1].  TL;DR "All of the
UTC+X UTC-X timezones could be translated into entries in
/usr/share/zoneinfo/Etc. For example, "UTC-5" would be converted to
TZ=:Etc/GMT+5"

Simmilar to Quozl's patch [2].  Quozl explains his patch as "translating
from ISO 8601 to POSIX.1 format, by reversing the sign"

[1]: http://lists.sugarlabs.org/archive/sugar-devel/2011-March/030532.html
[2]: http://lists.sugarlabs.org/archive/sugar-devel/2011-March/030497.html

extensions/cpsection/datetime/model.py

@@ -71,12 +71,28 @@ def print_timezone():
     print get_timezone()
 
 
+def fix_UTC_time_zone(timezone):
+    # Fixes the issue where the timezones are
+    # wrong when using UTC to set the time.
+    # This works by inverting the +/- and using
+    # the Etc/GMT... to be POSIX compliant.
+    if '+' in timezone:
+        new = timezone.replace('+', '-')
+    elif '-' in timezone:
+        new = timezone.replace('-', '+')
+    else:
+        new = 'UTC'
+    return 'Etc/' + new.replace('UTC', 'GMT')
+
+
 def set_timezone(timezone):
     """Set the system timezone
     timezone : e.g. 'America/Los_Angeles'
     """
     timezones = read_all_timezones()
     if timezone in timezones:
+        if timezone.startswith('UTC'):
+            timezone = fix_UTC_time_zone(timezone)
         os.environ['TZ'] = timezone
         settings = Gio.Settings('org.sugarlabs.date')
         settings.set_string('timezone', timezone)