diff --git a/library/Icinga/Web/Form/Element/DateTimePicker.php b/library/Icinga/Web/Form/Element/DateTimePicker.php
index 2173c6c8a..f48742ea6 100644
--- a/library/Icinga/Web/Form/Element/DateTimePicker.php
+++ b/library/Icinga/Web/Form/Element/DateTimePicker.php
@@ -112,6 +112,16 @@ class DateTimePicker extends FormElement
         return $this->max;
     }
 
+    /**
+     * Get the expected date and time format of any user input
+     *
+     * @return string
+     */
+    public function getFormat()
+    {
+        return $this->local ? 'Y-m-d\TH:i:s' : DateTime::RFC3339;
+    }
+
     /**
      * Is the date and time valid?
      *
@@ -127,7 +137,7 @@ class DateTimePicker extends FormElement
         }
 
         if (! $value instanceof DateTime) {
-            $format = $this->local === true ? 'Y-m-d\TH:i:s' : DateTime::RFC3339;
+            $format = $this->getFormat();
             $dateTime = DateTime::createFromFormat($format, $value);
             if ($dateTime === false) {
                 $dateTime = DateTime::createFromFormat(substr($format, 0, strrpos($format, ':')), $value);
diff --git a/modules/monitoring/application/forms/Command/Object/ScheduleHostDowntimeCommandForm.php b/modules/monitoring/application/forms/Command/Object/ScheduleHostDowntimeCommandForm.php
index 05d1451d9..046b6fee0 100644
--- a/modules/monitoring/application/forms/Command/Object/ScheduleHostDowntimeCommandForm.php
+++ b/modules/monitoring/application/forms/Command/Object/ScheduleHostDowntimeCommandForm.php
@@ -61,6 +61,22 @@ class ScheduleHostDowntimeCommandForm extends ScheduleServiceDowntimeCommandForm
      */
     public function onSuccess()
     {
+        $end = $this->getValue('end')->getTimestamp();
+        if ($end <= $this->getValue('start')->getTimestamp()) {
+            $endElement = $this->_elements['end'];
+            $endElement->setValue($endElement->getValue()->format($endElement->getFormat()));
+            $endElement->addError($this->translate('The end time must be greater than the start time'));
+            return false;
+        }
+
+        $now = new DateTime;
+        if ($end <= $now->getTimestamp()) {
+            $endElement = $this->_elements['end'];
+            $endElement->setValue($endElement->getValue()->format($endElement->getFormat()));
+            $endElement->addError($this->translate('A downtime must not be in the past'));
+            return false;
+        }
+
         foreach ($this->objects as $object) {
             /** @var \Icinga\Module\Monitoring\Object\Host $object */
             if (($childHostsEl = $this->getElement('child_hosts')) !== null) {
diff --git a/modules/monitoring/application/forms/Command/Object/ScheduleServiceDowntimeCommandForm.php b/modules/monitoring/application/forms/Command/Object/ScheduleServiceDowntimeCommandForm.php
index 9e6efb5ca..14ac85120 100644
--- a/modules/monitoring/application/forms/Command/Object/ScheduleServiceDowntimeCommandForm.php
+++ b/modules/monitoring/application/forms/Command/Object/ScheduleServiceDowntimeCommandForm.php
@@ -197,6 +197,22 @@ class ScheduleServiceDowntimeCommandForm extends ObjectsCommandForm
      */
     public function onSuccess()
     {
+        $end = $this->getValue('end')->getTimestamp();
+        if ($end <= $this->getValue('start')->getTimestamp()) {
+            $endElement = $this->_elements['end'];
+            $endElement->setValue($endElement->getValue()->format($endElement->getFormat()));
+            $endElement->addError($this->translate('The end time must be greater than the start time'));
+            return false;
+        }
+
+        $now = new DateTime;
+        if ($end <= $now->getTimestamp()) {
+            $endElement = $this->_elements['end'];
+            $endElement->setValue($endElement->getValue()->format($endElement->getFormat()));
+            $endElement->addError($this->translate('A downtime must not be in the past'));
+            return false;
+        }
+
         foreach ($this->objects as $object) {
             /** @var \Icinga\Module\Monitoring\Object\Service $object */
             $downtime = new ScheduleServiceDowntimeCommand();