Add the timezone property to app

It will make it easier to get a default time zone.

calsocial/__init__.py

@@ -80,6 +80,8 @@ class CalendarSocialApp(Flask):
 
         Flask.__init__(self, name)
 
+        self._timezone = None
+
         config_name = os.environ.get('ENV', config or 'dev')
         self.config.from_pyfile(f'config_{config_name}.py', True)
         # Make sure we look up users both by their usernames and email addresses
@@ -106,6 +108,32 @@ class CalendarSocialApp(Flask):
 
             self.route(*args, **kwargs)(attr)
 
+    @property
+    def timezone(self):
+        """The default time zone of the app
+        """
+
+        from warnings import warn
+
+        from flask import has_app_context
+        from pytz import timezone, utc
+        from pytz.exceptions import UnknownTimeZoneError
+
+        if not has_app_context():
+            return utc
+
+        if not self._timezone:
+            timezone_str = current_app.settings.get('DEFAULT_TIMEZONE', 'UTC')
+
+            try:
+                self._timezone = timezone(timezone_str)
+            except UnknownTimeZoneError:
+                warn(f'Timezone of {self} (or the default timezone) "{timezone_str}" is invalid')
+
+                self._timezone = utc
+
+        return self._timezone
+
     @staticmethod
     @route('/')
     def hello():