@niveditakumari  You only need to change below line of code

function onSubmit() {
    //Type appropriate comment here, and begin script below

    var validateSelectedDate = new GlideAjax('global.ValidationUtils');
    validateSelectedDate.addParam('sysparm_name', 'validateDate');
    validateSelectedDate.addParam('sysparm_expectedDate', g_form.getValue('expected_date')); //Replace your date time variable
    validateSelectedDate.getXMLAnswer(function getExpectedDate(answer) {
        var validatedOutput = JSON.parse(answer);

        if (validatedOutput.correctDateSelected == 'No') {
            g_form.addErrorMessage('kindly select date after ' + validatedOutput.requiredDate.toString());
            return false;
        }
    });


}

If I could help you with your Query then, please hit the Thumb Icon and mark as Correct !!