diff --git a/inc/RRule.php b/inc/RRule.php
index 2bde986e..cd8aeb1f 100644
--- a/inc/RRule.php
+++ b/inc/RRule.php
@@ -152,7 +152,9 @@ class Rfc5545Duration {
     if ( empty($this->as_text) ) {
       $this->as_text = ($this->epoch_seconds < 0 ? '-P' : 'P');
       $in_duration = abs($this->epoch_seconds);
-      if ( $in_duration >= 86400 ) {
+      if ( $in_duration == 0 ) {
+        $this->as_text .= '0D';
+      } elseif ( $in_duration >= 86400 ) {
         $this->days = floor($in_duration / 86400);
         $in_duration -= $this->days * 86400;
         if ( $in_duration == 0 && ($this->days / 7) == floor($this->days / 7) ) {