alert message 30 days before the expiration date

hello

i could to find a script to run auto fill date (current date) when you open the file called "todaysDate"

// variable to store whether document has been opened already or not

var bAlreadyOpened;

function docOpened()

{

if(bAlreadyOpened != "true")

{

// document has just been opened

var d = new Date();

var sDate = util.printd("mm/dd/yyyy", d);

                // set date now

                app.alert("About to insert date into field now");

this.getField("todaysDate").value = sDate;

// now set bAlreadyOpened to true so it doesn’t

// run again

bAlreadyOpened = "true";

}

else

{

// document has already been opened

}

}

// call the docOpened() function

docOpened();

but i want to get alarm message and red frame like the required fields on the expiration date field when the current date field becomes (within and after ) the last 30 days of the expiration date field "enddate" 

thank you in advance