=IF($F4="On Board",MAX(0,((S$1-$M4)/7))*Q4,"")

S1= 3/31/2010
M4= 10/15/2009
Q4= $573

How can I fix the formula above so it will correctly calculate the cost
based on a 2009 date, or a 2010 date. Currently, the formula only works if
M4 has a date of 2010.