diff --git a/src/IdentityCard.php b/src/IdentityCard.php
index dd5c22a..6b4d061 100644
--- a/src/IdentityCard.php
+++ b/src/IdentityCard.php
@@ -325,8 +325,8 @@ public function getConstellation(): string
 
 		$day = (int) substr(static::$idCard, 12, 2);
 
-		if ($day >= $this->constellationEdgeDays[$month]) {
-			$month = $month + 1;
+		if ($day < $this->constellationEdgeDays[$month]) {
+			$month = $month - 1;
 		}
 
 		if ($month >= 0) {