Added guess_timezone_by_javascript() function

Function is used to identify the timezone by the date string, returned
by JavaScript

timezones/test_timezones.py

@@ -42,5 +42,17 @@ def test_guess_timezone():
     assert tz_utils.guess_timezone_by_ip('127.0.0.1', only_name=True) == None
 
 
+def test_guess_timezone_by_javascript():
+    asia_yekaterinburg = tz_utils.get_timezone('Asia/Yekaterinburg')
+    for i in xrange(2):  # to ensure that cache breaks nothing
+        assert tz_utils.guess_timezone_by_javascript('Tue Feb 01 2005 00:00:00 GMT+0500 (YEKT)') == asia_yekaterinburg
+        assert tz_utils.guess_timezone_by_javascript('Mon Aug 01 2005 00:00:00 GMT+0600 (YEKST)') == asia_yekaterinburg
+        assert tz_utils.guess_timezone_by_javascript('Mon Aug 01 2005 00:00:00 GMT+0600 (OOOPS)') == None
+
+    assert tz_utils.guess_timezone_by_javascript('Tue Feb 01 2005 00:00:00 GMT+0500 (YEKT)', True) == 'Asia/Yekaterinburg'
+    assert tz_utils.guess_timezone_by_javascript('Mon Aug 01 2005 00:00:00 GMT+0600 (YEKST)', True) == 'Asia/Yekaterinburg'
+    assert tz_utils.guess_timezone_by_javascript('Mon Aug 01 2005 00:00:00 GMT+0600 (OOOPS)', True) == None
+
+
 def test_get_timezonez():
     assert len(list(zones.get_timezones(only_us=True))) == 8

timezones/tz_utils.py

@@ -86,6 +86,72 @@ def guess_timezone_by_ip(ip, only_name=False):
     return None
 
 
+# the map tzname -> timezone string (YEKT -> "Asia/Yekaterinburg",
+#                                    YEKST -> "Asia/Yekaterinburg" and so on)
+_tzname_cache = {}
+
+def guess_timezone_by_javascript(date_string, only_name=False):
+    """Given the `date_string` as returned by JavaScript, tries to return the
+    user browser timezone.
+
+    Example usage::
+
+        from timezones import tz_utils
+        assert tz_utils.guess_timezone_by_javascript('Tue Feb 01 2005 00:00:00 GMT+0500 (YEKT)', only_name=True) == 'Asia/Yekaterinburg'
+
+    You may get the javascript date representation as::
+
+        new Date().toString()
+
+    Implementation note: currently we ignore the tz offset component of the
+    date string object, and rely on the timezone name in parentheses only.
+    """
+
+    def obj_or_name(obj):
+        """ helper function returning timezone object or its name only"""
+        if obj and only_name:
+            return obj.zone
+        return obj
+
+    if not date_string:
+        return None
+
+    chunks = date_string.split()
+    datetime_info = ' '.join(chunks[:5])  # "Tue Feb 01 2005 00:00:00"
+    timezone_info = chunks[5:]  # ['GMT+0500', '(YEKT)']
+
+    if len(timezone_info) != 2:
+        return None
+
+    try:
+        dt = datetime.strptime(datetime_info, '%a %b %d %Y %H:%M:%S')
+    except ValueError:  # unable to convert string to datetime object
+        return None
+
+    expected_tzname = timezone_info[1].strip('()')
+
+    if expected_tzname in _tzname_cache:
+        # check in cache first (cache stores srings or None`s)
+        timezone_name = _tzname_cache[expected_tzname]
+        if not timezone_name:
+            return None
+        timezone_obj = pytz.timezone(timezone_name)
+        return obj_or_name(timezone_obj)
+
+    for timezone in pytz.all_timezones:
+        timezone_obj = pytz.timezone(timezone)
+        tzname = timezone_obj.tzname(dt)
+        _tzname_cache[tzname] = timezone
+        if tzname == expected_tzname:
+            break
+    else:
+        _tzname_cache[expected_tzname] = None
+        return None
+
+    return obj_or_name(timezone_obj)
+
+
+
 def get_timezone(tzname):
     """Get a timezone instance by name or return `None`.