fix: format date intially

packages/primevue/src/datepicker/DatePicker.vue

@@ -1251,7 +1251,7 @@ export default {
         },
         formatValue(value) {
             if (typeof value === 'string') {
-                return value;
+                return this.dateFormat ? this.formatDate(new Date(value), this.dateFormat) : value;
             }
 
             let formattedValue = '';