diff --git a/tests/test_sync.py b/tests/test_sync.py
new file mode 100644
index 0000000..6ef1f72
--- /dev/null
+++ b/tests/test_sync.py
@@ -0,0 +1,125 @@
+import datetime
+import hashlib
+import operator
+import unittest
+
+import dateutil.parser
+from pytz import timezone, utc
+
+from gcal_sync import CalendarSync
+
+
+class TestCalendarSync(unittest.TestCase):
+    @staticmethod
+    def sha1(string):
+        if isinstance(string, str):
+            string = string.encode('utf8')
+        h = hashlib.sha1()
+        h.update(string)
+        return h.hexdigest()
+
+    @staticmethod
+    def gen_events(start, stop, start_time):
+        one_hour = datetime.datetime(
+            1, 1, 1, 2) - datetime.datetime(1, 1, 1, 1)
+        for i in range(start, stop):
+            event_start = start_time + (one_hour * i)
+            event_end = event_start + one_hour
+            updated = utc.normalize(
+                event_start.astimezone(utc)).replace(tzinfo=None)
+            yield {
+                'summary': 'test event __ {}'.format(i),
+                'location': 'la la la {}'.format(i),
+                'description': 'test TEST -- test event {}'.format(i),
+                'start': {
+                    'dateTime': event_start.isoformat()
+                },
+                'end': {
+                    'dateTime': event_end.isoformat(),
+                },
+                "iCalUID": "{}@test.com".format(TestCalendarSync.sha1("test - event {}".format(i))),
+                "updated": updated.isoformat() + 'Z',
+                "created": updated.isoformat() + 'Z'
+            }
+
+    @staticmethod
+    def gen_list_to_compare(start, stop):
+        for i in range(start, stop):
+            yield {'iCalUID': 'test{}'.format(i)}
+
+    @staticmethod
+    def get_start_date(event):
+        event_start = event['start']
+        start_date = None
+        if 'date' in event_start:
+            start_date = event_start['date']
+        if 'dateTime' in event_start:
+            start_date = event_start['dateTime']
+        return dateutil.parser.parse(start_date)
+
+    def test_compare(self):
+        lst_src = list(TestCalendarSync.gen_list_to_compare(1, 5))
+        lst_dst = list(TestCalendarSync.gen_list_to_compare(3, 7))
+
+        to_ins, to_upd, to_del = CalendarSync._events_list_compare(
+            lst_src, lst_dst)
+
+        self.assertEqual(len(to_ins), 2)
+        self.assertEqual(len(to_upd), 2)
+        self.assertEqual(len(to_del), 2)
+
+        self.assertEqual(to_ins, lst_src[:2])
+        self.assertEqual(to_upd, list(zip(lst_src[2:4], lst_dst[:2])))
+        self.assertEqual(to_del, lst_dst[2:])
+
+    def test_filter_events_by_date(self):
+        msk = timezone('Europe/Moscow')
+        now = utc.localize(datetime.datetime.utcnow())
+        msk_now = msk.normalize(now.astimezone(msk))
+
+        one_hour = datetime.datetime(
+            1, 1, 1, 2) - datetime.datetime(1, 1, 1, 1)
+        date_cmp = msk_now + (one_hour * 5)
+
+        events = list(TestCalendarSync.gen_events(1, 11, msk_now))
+        events_pending = CalendarSync._filter_events_by_date(
+            events, date_cmp, operator.ge)
+        events_past = CalendarSync._filter_events_by_date(
+            events, date_cmp, operator.lt)
+
+        self.assertEqual(len(events_pending), 6)
+        self.assertEqual(len(events_past), 4)
+
+        for event in events_pending:
+            self.assertGreaterEqual(TestCalendarSync.get_start_date(event), date_cmp)
+
+        for event in events_past:
+            self.assertLess(TestCalendarSync.get_start_date(event), date_cmp)
+
+    def test_filter_events_to_update(self):
+        msk = timezone('Europe/Moscow')
+        now = utc.localize(datetime.datetime.utcnow())
+        msk_now = msk.normalize(now.astimezone(msk))
+
+        one_hour = datetime.datetime(
+            1, 1, 1, 2) - datetime.datetime(1, 1, 1, 1)
+        date_upd = msk_now + (one_hour * 5)
+
+        count = 10
+        events_old = list(TestCalendarSync.gen_events(1, 1 + count, msk_now))
+        events_new = list(TestCalendarSync.gen_events(1, 1 + count, date_upd))
+
+        sync1 = CalendarSync(None, None)
+        sync1.to_update = list(zip(events_new, events_old))
+        sync1._filter_events_to_update()
+
+        sync2 = CalendarSync(None, None)
+        sync2.to_update = list(zip(events_old, events_new))
+        sync2._filter_events_to_update()
+
+        self.assertEqual(len(sync1.to_update), count)
+        self.assertEqual(len(sync2.to_update), 0)
+
+
+if __name__ == '__main__':
+    unittest.main()