The Error: “INVALID_DATE_VALUE_MUST_BE_1”
This error displays while entering the Invalid date format value in the external suitelet date field. we can’t be able to customize this error using client script, because it’s a standard NetSuite error before the client script validates the fields this NetSuite standard error populates.
And also, we can’t be able to Change or customize the Error message because it’s a system error.