--- include/classes/parsers/CRelativeTimeParser.php.bak	2022-03-29 00:13:41.903955086 +0900
+++ include/classes/parsers/CRelativeTimeParser.php	2022-04-01 00:06:45.815977824 +0900
@@ -199,7 +199,25 @@
 						'y' => 'year'
 					];
 
-					$date->modify($token['sign'].$token['value'].' '.$units[$token['suffix']]);
+					if ($token['suffix'] === 'M') {
+						/* correctly calculates end of months
+						 * see the example in https://www.php.net/manual/en/datetime.modify.php */
+						$date0 = clone($date);
+						$date1 = clone($date);
+						$date2 = clone($date);
+						$date->modify($token['sign'].$token['value'].' '.$units[$token['suffix']]);
+						$date1->modify($token['sign'].$token['value'].' '.$units[$token['suffix']]);
+						$date1->modify(-1 * intval($token['sign'].$token['value']).' '.$units[$token['suffix']]);
+						if ($date1 != $date2) {
+							$date->modify('last day of last month');
+						}
+						elseif ($date0 == $date2->modify('last day of this month')) {
+							$date->modify('last day of this month');
+						}
+					}
+					else {
+						$date->modify($token['sign'].$token['value'].' '.$units[$token['suffix']]);
+					}
 					break;
 			}
 		}