diff --git a/main.go b/main.go
index 7bc487a..84be626 100644
--- a/main.go
+++ b/main.go
@@ -61,6 +61,11 @@ func main() {
 	for i := 0; i < *days; i++ {
 		date := now.Add(time.Duration(i * 24) * time.Hour)
 
+		eventMap, err := getDayEvents(srv, myCal.Id, date)
+		if err != nil {
+			log.Fatal(err)
+		}
+
 		for _, class := range classes {
 			if !class.happensOnDay(date) {
 				continue
@@ -68,54 +73,88 @@ func main() {
 
 			ev := class.buildEvent(date)
 
-			_, err := srv.Events.Insert(myCal.Id, ev).Do()
-			if err != nil {
-				log.Fatal(err)
+			old := eventMap[ev.Summary]
+
+			if old == nil {
+				log.Printf("add: [%s] %s", ev.Start.DateTime, ev.Summary)
+				_, err := srv.Events.Insert(myCal.Id, ev).Do()
+				if err != nil {
+					log.Fatal(err)
+				}
+			} else {
+				if !eventsEqual(old, ev) {
+					log.Printf("upd: [%s] %s", ev.Start.DateTime, ev.Summary)
+					_, err := srv.Events.Update(myCal.Id, old.Id, ev).Do()
+					if err != nil {
+						log.Fatal(err)
+					}
+				}
 			}
 		}
 	}
+}
 
+func getDayEvents(srv *calendar.Service, calendarId string, t time.Time) (map[string]*calendar.Event, error) {
+	start := time.Date(
+		t.Year(),
+		t.Month(),
+		t.Day(),
+		0, 0, 0, 0,
+		t.Location())
 
-	/*
-	ev := &calendar.Event{
-		Summary: "Test event",
-		Start: &calendar.EventDateTime{
-			DateTime: "2020-09-09T16:00:00",
-			TimeZone: "US/Pacific",
-		},
-		End: &calendar.EventDateTime{
-			DateTime: "2020-09-09T17:00:00",
-			TimeZone: "US/Pacific",
-		},
-		Attendees: []*calendar.EventAttendee{
-			&calendar.EventAttendee{
-				Email: "oliver.park@heliosns.org",
-			},
-		},
-	}
+	// This may be wrong for DST changes, but it's the middle of the night and probably doesn't matter
+	end := start.Add(24 * time.Hour)
 
-	res, err := srv.Events.Insert(myCal.Id, ev).Do()
+	events, err := srv.Events.List(calendarId).
+		ShowDeleted(false).
+		SingleEvents(true).
+		TimeMin(start.Format(time.RFC3339)).
+		TimeMax(end.Format(time.RFC3339)).
+		MaxResults(250).
+		Do()
 	if err != nil {
-		log.Fatal(err)
+		return nil, err
 	}
 
-	log.Printf("%#v", res)
-	*/
-
-	/*
-	t := time.Now().Format(time.RFC3339)
-	events, err := srv.Events.List(myCal.Id).ShowDeleted(false).
-		SingleEvents(true).TimeMin(t).MaxResults(250).OrderBy("startTime").Do()
-	if err != nil {
-		log.Fatal(err)
-	}
+	eventMap := map[string]*calendar.Event{}
 
 	for _, item := range events.Items {
-		date := item.Start.DateTime
-		if date == "" {
-			date = item.Start.Date
-		}
-		log.Printf("%v (%v)", item.Summary, date)
+		eventMap[item.Summary] = item
 	}
-	*/
+
+	return eventMap, nil
+}
+
+func eventsEqual(a *calendar.Event, b *calendar.Event) bool {
+	if a.Summary != b.Summary {
+		return false
+	}
+
+	if a.Description != b.Description {
+		return false
+	}
+
+	if a.Start.DateTime[:19] != b.Start.DateTime[:19] {
+		return false
+	}
+
+	if a.End.DateTime[:19] != b.End.DateTime[:19] {
+		return false
+	}
+
+	if len(a.Attendees) != len(b.Attendees) {
+		return false
+	}
+
+	emails := map[string]bool{}
+	for _, attendee := range a.Attendees {
+		emails[attendee.Email] = true
+	}
+	for _, attendee := range b.Attendees {
+		if !emails[attendee.Email] {
+			return false
+		}
+	}
+
+	return true
 }