diff --git a/fhem/CHANGED b/fhem/CHANGED
index 22f3627fc..1b09d2884 100644
--- a/fhem/CHANGED
+++ b/fhem/CHANGED
@@ -1,5 +1,6 @@
 # Add changes at the top of the list. Keep it in ASCII, and 80-char wide.
 # Do not insert empty lines here, update check depends on it.
+  - feature: 57_Calendar: deal with non-existent end times
   - bugfix: SOMFY: fix non-working on/off-for-timer methods
                    made positioning attributes optional
   - feature: SOMFY: support for exact positioning (one-time setup of run times required)
diff --git a/fhem/FHEM/57_Calendar.pm b/fhem/FHEM/57_Calendar.pm
index 8b10800a9..f999e7677 100644
--- a/fhem/FHEM/57_Calendar.pm
+++ b/fhem/FHEM/57_Calendar.pm
@@ -622,12 +622,18 @@ sub isAlarmed {
 sub isStarted {
   my ($self,$t) = @_;
   return 0 if($self->isDeleted());
-  return $self->{start}<= $t && $t< $self->{end} ? 1 : 0;
+  return 0 unless(defined($self->{start}));
+  return 0 if($t < $self->{start});
+  if(defined($self->{end})) {
+    return 0 if($t>= $self->{end});
+  }
+  return 1;
 }
 
 sub isEnded {
   my ($self,$t) = @_;
   return 0 if($self->isDeleted());
+  return 0 unless(defined($self->{end}));
   return $self->{end}<= $t ? 1 : 0;
 }