looping a calculation until workbook closes
 

OK, cell A1 has a date and time in the future and cell A2 has "=NOW()". Cell
A3 has A1-A2 to get a countdown of sorts. The only problem is that it only
calculates when the workbook is first opened or data is altered in the
workbook. How do I get it to loop the "Calculate" command continuously while
the workbook is open? So it has a countdown that is actually counting down
while you're looking at it? I don't plan on doing anything else with the
workbook besides use it as a countdown timer.