diff --git a/src/applications/calendar/editor/PhabricatorCalendarEventEditEngine.php b/src/applications/calendar/editor/PhabricatorCalendarEventEditEngine.php
--- a/src/applications/calendar/editor/PhabricatorCalendarEventEditEngine.php
+++ b/src/applications/calendar/editor/PhabricatorCalendarEventEditEngine.php
@@ -212,4 +212,45 @@
     return $fields;
   }
 
+  protected function willBuildEditForm($object, array $fields) {
+    $all_day_field = idx($fields, 'isAllDay');
+    $start_field = idx($fields, 'start');
+    $end_field = idx($fields, 'end');
+
+    if ($all_day_field) {
+      $is_all_day = $all_day_field->getValueForTransaction();
+
+      $control_ids = array();
+      if ($start_field) {
+        $control_ids[] = $start_field->getControlID();
+      }
+      if ($end_field) {
+        $control_ids[] = $end_field->getControlID();
+      }
+
+      Javelin::initBehavior(
+        'event-all-day',
+        array(
+          'allDayID' => $all_day_field->getControlID(),
+          'controlIDs' => $control_ids,
+        ));
+
+    } else {
+      $is_all_day = $object->getIsAllDay();
+    }
+
+    if ($is_all_day) {
+      if ($start_field) {
+        $start_field->setHideTime(true);
+      }
+
+      if ($end_field) {
+        $end_field->setHideTime(true);
+      }
+    }
+
+
+
+    return $fields;
+  }
 }
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngine.php b/src/applications/transactions/editengine/PhabricatorEditEngine.php
--- a/src/applications/transactions/editengine/PhabricatorEditEngine.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngine.php
@@ -1176,6 +1176,8 @@
     $controller = $this->getController();
     $request = $controller->getRequest();
 
+    $fields = $this->willBuildEditForm($object, $fields);
+
     $form = id(new AphrontFormView())
       ->setUser($viewer)
       ->addHiddenInput('editEngine', 'true');
@@ -1210,6 +1212,10 @@
     return $form;
   }
 
+  protected function willBuildEditForm($object, array $fields) {
+    return $fields;
+  }
+
   private function buildEditFormActionButton($object) {
     if (!$this->isEngineConfigurable()) {
       return null;
diff --git a/src/applications/transactions/editfield/PhabricatorEpochEditField.php b/src/applications/transactions/editfield/PhabricatorEpochEditField.php
--- a/src/applications/transactions/editfield/PhabricatorEpochEditField.php
+++ b/src/applications/transactions/editfield/PhabricatorEpochEditField.php
@@ -4,6 +4,7 @@
   extends PhabricatorEditField {
 
   private $allowNull;
+  private $hideTime;
 
   public function setAllowNull($allow_null) {
     $this->allowNull = $allow_null;
@@ -14,9 +15,19 @@
     return $this->allowNull;
   }
 
+  public function setHideTime($hide_time) {
+    $this->hideTime = $hide_time;
+    return $this;
+  }
+
+  public function getHideTime() {
+    return $this->hideTime;
+  }
+
   protected function newControl() {
     return id(new AphrontFormDateControl())
       ->setAllowNull($this->getAllowNull())
+      ->setIsTimeDisabled($this->getHideTime())
       ->setViewer($this->getViewer());
   }
 
@@ -26,9 +37,7 @@
   }
 
   protected function newConduitParameterType() {
-    // TODO: This isn't correct, but we don't have any methods which use this
-    // yet.
-    return new ConduitIntParameterType();
+    return new ConduitEpochParameterType();
   }
 
 }
diff --git a/webroot/rsrc/js/application/calendar/behavior-event-all-day.js b/webroot/rsrc/js/application/calendar/behavior-event-all-day.js
--- a/webroot/rsrc/js/application/calendar/behavior-event-all-day.js
+++ b/webroot/rsrc/js/application/calendar/behavior-event-all-day.js
@@ -2,15 +2,16 @@
  * @provides javelin-behavior-event-all-day
  */
 
-
 JX.behavior('event-all-day', function(config) {
-  var checkbox = JX.$(config.allDayID);
-  JX.DOM.listen(checkbox, 'change', null, function() {
-    var start = JX.$(config.startDateID);
-    var end = JX.$(config.endDateID);
+  var all_day = JX.$(config.allDayID);
+
+  JX.DOM.listen(all_day, 'change', null, function() {
+    var is_all_day = !!parseInt(all_day.value, 10);
 
-    JX.DOM.alterClass(start, 'no-time', checkbox.checked);
-    JX.DOM.alterClass(end, 'no-time', checkbox.checked);
+    for (var ii = 0; ii < config.controlIDs.length; ii++) {
+      var control = JX.$(config.controlIDs[ii]);
+      JX.DOM.alterClass(control, 'no-time', is_all_day);
+    }
   });
 
 });