diff --git a/src/applications/calendar/xaction/PhabricatorCalendarEventFrequencyTransaction.php b/src/applications/calendar/xaction/PhabricatorCalendarEventFrequencyTransaction.php
index 33c954ac9f..6fb8ae8f31 100644
--- a/src/applications/calendar/xaction/PhabricatorCalendarEventFrequencyTransaction.php
+++ b/src/applications/calendar/xaction/PhabricatorCalendarEventFrequencyTransaction.php
@@ -1,114 +1,141 @@
 <?php
 
 final class PhabricatorCalendarEventFrequencyTransaction
   extends PhabricatorCalendarEventTransactionType {
 
   const TRANSACTIONTYPE = 'calendar.frequency';
 
   public function generateOldValue($object) {
     $rrule = $object->newRecurrenceRule();
 
     if (!$rrule) {
       return null;
     }
 
     return $rrule->getFrequency();
   }
 
   public function applyInternalEffects($object, $value) {
     $rrule = id(new PhutilCalendarRecurrenceRule())
       ->setFrequency($value);
 
+    // If the user creates a monthly event on the 29th, 30th or 31st of a
+    // month, it means "the 30th of every month" as far as the RRULE is
+    // concerned. Such an event will not occur on months with fewer days.
+
+    // This is surprising, and proably not what the user wants. Instead,
+    // schedule these events relative to the end of the month: on the "-1st",
+    // "-2nd" or "-3rd" day of the month. For example, a monthly event on
+    // the 31st of a 31-day month translates to "every month, on the last
+    // day of the month".
+    if ($value == PhutilCalendarRecurrenceRule::FREQUENCY_MONTHLY) {
+      $start_datetime = $object->newStartDateTime();
+
+      $y = $start_datetime->getYear();
+      $m = $start_datetime->getMonth();
+      $d = $start_datetime->getDay();
+      if ($d >= 29) {
+        $year_map = PhutilCalendarRecurrenceRule::getYearMap(
+          $y,
+          PhutilCalendarRecurrenceRule::WEEKDAY_MONDAY);
+
+        $month_days = $year_map['monthDays'][$m];
+        $schedule_on = -(($month_days + 1) - $d);
+
+        $rrule->setByMonthDay(array($schedule_on));
+      }
+    }
+
     $object->setRecurrenceRule($rrule);
   }
 
   public function validateTransactions($object, array $xactions) {
     $errors = array();
 
     $valid = array(
       PhutilCalendarRecurrenceRule::FREQUENCY_DAILY,
       PhutilCalendarRecurrenceRule::FREQUENCY_WEEKLY,
       PhutilCalendarRecurrenceRule::FREQUENCY_MONTHLY,
       PhutilCalendarRecurrenceRule::FREQUENCY_YEARLY,
     );
     $valid = array_fuse($valid);
 
     foreach ($xactions as $xaction) {
       $value = $xaction->getNewValue();
 
       if (!isset($valid[$value])) {
         $errors[] = $this->newInvalidError(
           pht(
             'Event frequency "%s" is not valid. Valid frequences are: %s.',
             $value,
             implode(', ', $valid)),
           $xaction);
       }
     }
 
     return $errors;
   }
 
   public function getTitle() {
     $frequency = $this->getFrequency($this->getNewValue());
     switch ($frequency) {
       case PhutilCalendarRecurrenceRule::FREQUENCY_DAILY:
         return pht(
           '%s set this event to repeat daily.',
           $this->renderAuthor());
       case PhutilCalendarRecurrenceRule::FREQUENCY_WEEKLY:
         return pht(
           '%s set this event to repeat weekly.',
           $this->renderAuthor());
       case PhutilCalendarRecurrenceRule::FREQUENCY_MONTHLY:
         return pht(
           '%s set this event to repeat monthly.',
           $this->renderAuthor());
       case PhutilCalendarRecurrenceRule::FREQUENCY_YEARLY:
         return pht(
           '%s set this event to repeat yearly.',
           $this->renderAuthor());
     }
   }
 
   public function getTitleForFeed() {
     $frequency = $this->getFrequency($this->getNewValue());
     switch ($frequency) {
       case PhutilCalendarRecurrenceRule::FREQUENCY_DAILY:
         return pht(
           '%s set %s to repeat daily.',
           $this->renderAuthor(),
           $this->renderObject());
       case PhutilCalendarRecurrenceRule::FREQUENCY_WEEKLY:
         return pht(
           '%s set %s to repeat weekly.',
           $this->renderAuthor(),
           $this->renderObject());
       case PhutilCalendarRecurrenceRule::FREQUENCY_MONTHLY:
         return pht(
           '%s set %s to repeat monthly.',
           $this->renderAuthor(),
           $this->renderObject());
       case PhutilCalendarRecurrenceRule::FREQUENCY_YEARLY:
         return pht(
           '%s set %s to repeat yearly.',
           $this->renderAuthor(),
           $this->renderObject());
     }
   }
 
   private function getFrequency($value) {
     // NOTE: This is normalizing three generations of these transactions
     // to use RRULE constants. It would be vaguely nice to migrate them
     // for consistency.
 
     if (is_array($value)) {
       $value = idx($value, 'rule');
     } else {
       $value = $value;
     }
 
     return phutil_utf8_strtoupper($value);
   }
 
 }