diff --git a/php/PublicHolidayCalendar.php b/php/PublicHolidayCalendar.php
index e853f1a..c1adcfb 100644
--- a/php/PublicHolidayCalendar.php
+++ b/php/PublicHolidayCalendar.php
@@ -295,7 +295,7 @@ class PublicHolidayCalendar {
     private static function calculateGrandMotherDay (int $year): DateTimeImmutable {
         $grandMotherDay = static::createDate($year, 3, 1);
         try {
-            $grandMotherDay = $grandMotherDay->sub(new DateInterval('P' . (7 - (int)$grandMotherDay->format('N')) . 'D'));
+            $grandMotherDay = $grandMotherDay->add(new DateInterval('P' . (7 - (int)$grandMotherDay->format('N')) . 'D'));
         }
         catch (Exception) {
         }