diff --git a/htdocs/DateRangePicker.vue b/htdocs/DateRangePicker.vue
index 5a3763b..71e130c 100644
--- a/htdocs/DateRangePicker.vue
+++ b/htdocs/DateRangePicker.vue
@@ -53,8 +53,8 @@
 	}
 
 	function timeshift_month(date, diff) {
-		var month = date.getMonth() + diff;
-		return new Date(date.getFullYear() + Math.floor(month/12), month >= 0 ? (month%12) : (month%12 + 12), date.getDate());
+		var month = date.getMonth() + diff, month_mod12 = month%12;
+		return new Date(date.getFullYear() + Math.floor(month/12), month_mod12 >= 0 ? (month_mod12) : (month_mod12 + 12), date.getDate());
 	}
 
 	function datecalc(date, range, direction) {