diff --git a/resources/celerity/map.php b/resources/celerity/map.php
--- a/resources/celerity/map.php
+++ b/resources/celerity/map.php
@@ -481,7 +481,7 @@
     'rsrc/js/core/behavior-detect-timezone.js' => '78bc5d94',
     'rsrc/js/core/behavior-device.js' => 'ac2b1e01',
     'rsrc/js/core/behavior-drag-and-drop-textarea.js' => '7ad020a5',
-    'rsrc/js/core/behavior-fancy-datepicker.js' => '956f3eeb',
+    'rsrc/js/core/behavior-fancy-datepicker.js' => '36821f8d',
     'rsrc/js/core/behavior-form.js' => '55d7b788',
     'rsrc/js/core/behavior-gesture.js' => 'b58d1a2a',
     'rsrc/js/core/behavior-global-drag-and-drop.js' => '1cab0e9a',
@@ -635,7 +635,7 @@
     'javelin-behavior-editengine-reorder-configs' => '4842f137',
     'javelin-behavior-editengine-reorder-fields' => '0ad8d31f',
     'javelin-behavior-event-all-day' => '0b1bc990',
-    'javelin-behavior-fancy-datepicker' => '956f3eeb',
+    'javelin-behavior-fancy-datepicker' => '36821f8d',
     'javelin-behavior-global-drag-and-drop' => '1cab0e9a',
     'javelin-behavior-harbormaster-log' => 'b347a301',
     'javelin-behavior-herald-rule-editor' => '0922e81d',
@@ -1229,6 +1229,13 @@
       'aphront-typeahead-control-css',
       'phui-tag-view-css',
     ),
+    '36821f8d' => array(
+      'javelin-behavior',
+      'javelin-util',
+      'javelin-dom',
+      'javelin-stratcom',
+      'javelin-vector',
+    ),
     '3829a3cf' => array(
       'javelin-behavior',
       'javelin-uri',
@@ -1781,13 +1788,6 @@
       'javelin-vector',
       'javelin-stratcom',
     ),
-    '956f3eeb' => array(
-      'javelin-behavior',
-      'javelin-util',
-      'javelin-dom',
-      'javelin-stratcom',
-      'javelin-vector',
-    ),
     '9623adc1' => array(
       'javelin-behavior',
       'javelin-stratcom',
diff --git a/webroot/rsrc/js/core/behavior-fancy-datepicker.js b/webroot/rsrc/js/core/behavior-fancy-datepicker.js
--- a/webroot/rsrc/js/core/behavior-fancy-datepicker.js
+++ b/webroot/rsrc/js/core/behavior-fancy-datepicker.js
@@ -424,6 +424,13 @@
             value_m += 12;
             value_y--;
           }
+          // This relies on months greater than 11 rolling over into the next
+          // year and days less than 1 rolling back into the previous month.
+          var last_date = new Date(value_y, value_m, 0);
+          if (value_d > last_date.getDate()) {
+            // The date falls outside the new month, so stuff it back in.
+            value_d = last_date.getDate();
+          }
           break;
         case 'd':
           // User clicked a day.