You can use an OnTime procedure.  See:

http://www.cpearson.com/excel/ontime.htm

As an aside, you should try to avoid overuse of the Now() function in your
worksheet, ideally use a named range to refer to a single cell which has this
in or directly define a name such as "TimeDateNow" as NOW().
Use this range instead in the rest of the formulae in your workbook and it
will have far less overhead than lots of repetitive NOW functions.