set date and time with drop down

Try this.

NWF$(document).ready(function()
{
 NWF$('#'+dropdown).change(function()
 {
  fnSetDate();
 });
});
function fnSetDate()
{
 var dropdownvalue = NWF$('#'+dropdown).val();
 alert(dropdownvalue);
 if(dropdownvalue == 'Completed')
 {
 var fullDate = new Date();
 var twoDigitMonth = ("0" + (fullDate.getMonth() + 1)).slice(-2);
 var hours = fullDate.getHours();
 var minutes = fullDate.getMinutes();
 var currentDate = fullDate.getDate() + "/" + twoDigitMonth + "/" + fullDate.getFullYear();
 if (hours < 10) { hours = "0" + hours; }
 minutes = minutes - 2.5;
 minutes = 5 * Math.round( minutes / 5 );
 NWF$('#' + fldDateClosed).val(currentDate);

 NWF$('#' + fldDateClosed+ " + IMG + SELECT").val('10 AM');
 NWF$('#' + fldDateClosed+ " + IMG + SELECT + SELECT").val('45');
 }
}

Yes you need to copy this code in 'Form Settings' under 'Custom JavaScript'. When you change 'Status' drop down value based on selected value this will set current date and time to your date control.

Give names like 'fldDateClosed' to 'Date control' and 'dropdown' to 'Status' like below in your control settings.