diff --git a/wire/modules/Inputfield/InputfieldDatetime/types/InputfieldDatetimeSelect.php b/wire/modules/Inputfield/InputfieldDatetime/types/InputfieldDatetimeSelect.php
index 3ee1e5f2..217ee892 100644
--- a/wire/modules/Inputfield/InputfieldDatetime/types/InputfieldDatetimeSelect.php
+++ b/wire/modules/Inputfield/InputfieldDatetime/types/InputfieldDatetimeSelect.php
@@ -90,7 +90,7 @@ class InputfieldDatetimeSelect extends InputfieldDatetimeType {
 
 		for($n = 1; $n <= 12; $n++) {
 			$monthFormat = $abbreviate ? '%b' : '%B';
-			$monthLabel = $sanitizer->entities(strftime($monthFormat, mktime(0, 0, 0, $n)));
+			$monthLabel = $sanitizer->entities(strftime($monthFormat, mktime(0, 0, 0, $n, 1)));
 			$months->addOption($n, $monthLabel);
 		}