diff --git a/icalevents/icalparser.py b/icalevents/icalparser.py
index 66a6577..69beeba 100644
--- a/icalevents/icalparser.py
+++ b/icalevents/icalparser.py
@@ -97,8 +97,6 @@ def copy_to(self, new_start=None, uid=None):
         :param uid: UID of new event
         :return: new event
         """
-        duration = self.end - self.start
-
         if not new_start:
             new_start = self.start
 
@@ -109,7 +107,11 @@ def copy_to(self, new_start=None, uid=None):
         ne.summary = self.summary
         ne.description = self.description
         ne.start = new_start
-        ne.end = (new_start + duration)
+        
+        if self.end:
+            duration = self.end - self.start
+            ne.end = (new_start + duration)
+
         ne.all_day = (self.all_day and (new_start - self.start).seconds == 0)
         ne.uid = uid
 
@@ -131,10 +133,10 @@ def create_event(component, tz=UTC):
     
     if component.get('dtend'):
         event.end = normalize(component.get('dtend').dt, tz=tz)
-    elif component.get('duration'):
+    elif component.get('duration'): # compute implicit end as start + duration
         event.end = event.start + component.get('duration').dt
-    else:
-        raise ValueError("Event has neither end, nor duration property.")
+    else: # compute implicit end as start + 0
+        event.end = event.start
     
     event.summary = str(component.get('summary'))
     event.description = str(component.get('description'))
diff --git a/test/test_data/duration.ics b/test/test_data/duration.ics
new file mode 100644
index 0000000..b686406
--- /dev/null
+++ b/test/test_data/duration.ics
@@ -0,0 +1,19 @@
+BEGIN:VCALENDAR
+BEGIN:VEVENT
+DTSTART:20180110
+DURATION:P3D
+DESCRIPTION:Event with duration (3 days), instead of explicit end.
+SUMMARY:Duration Event
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20180115T100000
+DURATION:PT3H
+DESCRIPTION:Event with duration (3 hours), instead of explicit end.
+SUMMARY:Duration Event
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20180120T120000
+DESCRIPTION:Event without explicit dtend, nor duration property.
+SUMMARY:Short event
+END:VEVENT
+END:VCALENDAR
diff --git a/test/test_data/recurring.ics b/test/test_data/recurring.ics
index 1ceb0ff..7c903fd 100644
--- a/test/test_data/recurring.ics
+++ b/test/test_data/recurring.ics
@@ -5,9 +5,18 @@ END:VTIMEZONE
 BEGIN:VEVENT
 DTSTART;TZID=Europe/Berlin:20181003T100000
 DTEND;TZID=Europe/Berlin:20181003T120000
-DESCRIPTION:Event recurring on wednesday each week
-SUMARRY:Recurring event
+DESCRIPTION:Event recurring on wednesday each week, except on 2018-10-29
+SUMARRY:Recurring Event
 RRULE:FREQ=WEEKLY;BYDAY=MO
 EXDATE;TZID=Europe/Berlin:20181029T100000
 END:VEVENT
+BEGIN:VEVENT
+DTSTART;TZID=Europe/Berlin:20180601T100000
+DTEND;TZID=Europe/Berlin:20180601T120000
+DESCRIPTION:Event recurring on friday each week, except on 2018-06-08/22
+SUMARRY:Recurring Event
+RRULE:FREQ=WEEKLY;BYDAY=FR
+EXDATE;TZID=Europe/Berlin:20180608T100000
+EXDATE;TZID=Europe/Berlin:20180622T100000
+END:VEVENT
 END:VCALENDAR
diff --git a/test/test_icalevents.py b/test/test_icalevents.py
index b7ad17b..f9ee2e6 100644
--- a/test/test_icalevents.py
+++ b/test/test_icalevents.py
@@ -1,7 +1,10 @@
 import unittest
 from icalevents import icalevents
-from datetime import date, timedelta
+from datetime import date, timedelta, datetime
 from time import sleep
+from dateutil.relativedelta import relativedelta
+from dateutil.tz import UTC
+from re import search
 
 
 class ICalEventsTests(unittest.TestCase):
@@ -31,6 +34,25 @@ def test_events(self):
 
         self.assertEqual(len(evs), 2, "two events are found")
         
+    def test_events_duration(self):
+        ical = "test/test_data/duration.ics"
+        start = date(2018, 1, 1)
+        end = date(2018, 2, 1)
+        
+        evs = icalevents.events(file=ical, start=start, end=end)
+        
+        e1 = evs[0]
+        self.assertEqual(e1.start.day, 10, "explicit event start")
+        self.assertEqual(e1.end.day, 13, "implicit event end")
+
+        e2 = evs[1]
+        self.assertEqual(e2.start.hour, 10, "explicit event start")
+        self.assertEqual(e2.end.hour, 13, "implicit event end")
+
+        e3 = evs[2]
+        self.assertEqual(e3.start.hour, 12, "explicit event start")
+        self.assertEqual(e3.end.hour, 12, "implicit event end")
+        
     def test_events_recurring(self):
         ical = "test/test_data/recurring.ics"
         start = date(2018, 10, 15)
@@ -47,6 +69,17 @@ def test_events_recurring(self):
         self.assertEqual(e2.start.tzinfo.utcoffset(e2.start), timedelta(seconds=3600), "check UTC offset without DST")
         
         self.assertEqual(e2.start.day, 5, "Check observance of exdate.")
+        
+    def test_events_exdates(self):
+        ical = "test/test_data/recurring.ics"
+        start = date(2018, 6, 1)
+        end = date(2018, 6, 30)
+        
+        evs = icalevents.events(file=ical, start=start, end=end)
+        
+        self.assertEqual(evs[0].start.day, 1, "check first recurrence.")
+        self.assertEqual(evs[1].start.day, 15, "check first exdate.")
+        self.assertEqual(evs[2].start.day, 29, "check second exdate.")
 
     def test_event_attributes(self):
         ical = "test/test_data/basic.ics"
@@ -111,3 +144,23 @@ def test_string_data(self):
 
         self.assertTrue(icalevents.all_done(key), "request is finished")
         self.assertEqual(len(icalevents.latest_events(key)), 2, "two events are found")
+        
+    def test_event_str(self):
+        ical = "test/test_data/duration.ics"
+        start = date(2018, 1, 1)
+        end = date(2018, 2, 1)
+        n = datetime.now(UTC)
+        m = relativedelta(hour=0, minute=0, second=0, microsecond=0)
+        
+        evs = icalevents.events(file=ical, start=start, end=end)
+        
+        e1 = evs[0]
+        self.assertIsNotNone(search(r"ended", str(e1.copy_to(n - relativedelta(days=5) + m))), "stringify past event")
+        self.assertIsNotNone(search(r"today", str(e1.copy_to(n - relativedelta(days=1) + m))), "stringify ongoing event")
+        self.assertIsNotNone(search(r"days left", str(e1.copy_to(n + relativedelta(days=3) + m))), "stringify future event")
+        
+        e2 = evs[1]
+        self.assertIsNotNone(search(r"ended", str(e2.copy_to(n - relativedelta(hours=5)))), "stringify past event")
+        self.assertIsNotNone(search(r"now", str(e2.copy_to(n - relativedelta(hours=1)))), "stringify ongoing event")
+        self.assertIsNotNone(search(r"hours left", str(e2.copy_to(n + relativedelta(hours=3)))), "stringify future event")
+        self.assertIsNotNone(search(r"days left", str(e2.copy_to(n + relativedelta(days=3)))), "stringify future event")
diff --git a/test/test_icalparser.py b/test/test_icalparser.py
index c95745a..6b6b1a4 100644
--- a/test/test_icalparser.py
+++ b/test/test_icalparser.py
@@ -39,6 +39,7 @@ def test_time_left(self):
     def test_event_copy_to(self):
         new_start = datetime(year=2017, month=2, day=5, hour=12, minute=5, tzinfo=UTC)
         eventC = self.eventA.copy_to(new_start)
+        new_uid = 1234567890
 
         self.assertNotEqual(eventC.uid, self.eventA.uid, "new event has new UID")
         self.assertEqual(eventC.start, new_start, "new event has new start")
@@ -47,8 +48,8 @@ def test_event_copy_to(self):
         self.assertEqual(eventC.summary, self.eventA.summary, "copy to: summary")
         self.assertEqual(eventC.description, self.eventA.description, "copy to: description")
 
-        eventD = eventC.copy_to()
-        self.assertNotEqual(eventD.uid, eventC.uid, "new event has new UID")
+        eventD = eventC.copy_to(uid=new_uid)
+        self.assertEqual(eventD.uid, new_uid, "new event has specified UID")
         self.assertEqual(eventD.start, eventC.start, "new event has same start")
         self.assertEqual(eventD.end, eventC.end, "new event has same end")
         self.assertEqual(eventD.all_day, eventC.all_day, "new event is no all day event")
@@ -84,3 +85,6 @@ def test_normalize(self):
         self.assertEqual(3, norm.second, "second")
         self.assertEqual(0, norm.microsecond, "microsecond")
         self.assertEqual(UTC, norm.tzinfo, "timezone")
+        
+        with self.assertRaises(ValueError, msg="type check effective"):
+            icalevents.icalparser.normalize(None)