How to use date condition in Dead line monitoring in workflow

Hi Guys,

I have to copy & modify leave request workflow (WS12300111) . if employee sent a mail to approver for his leave request . If approver didn't see mail and leave date is reached , leave should be "Approved" automatically. How to do ?. using dead line monitoring ? .

If example

employee requested leave from 31.12.2007 to 4.1.2008 but approver didn't see mail till 30.12.2007 11:59 then leave should be "Approved" automatically.

Thanks in advance .