GET A DAYTE SGOEN TO ACT LIKE ANOTHER DATE

Hello,
hi all
here is my question/problem

i have a calculator(based on a six day week) that i have working almost correctly
it calculates based on certain userform options(personal rates,dependent rates, child rates) picked will show for example(easiest one)

year(a1) from date(b1) to date(c1) days(d1) rate(e1) total(f1)
1. 2008 25/12/2008 31/12/2008 6 197.80 197.80
a2 b2 c2 d2 e2 f2
2. 2009 1/1/2009 7/1/2009 6 204.30 204.30

the print screen shows all the correct answers no matter what is picked on the userform if the dates are a cross over like above
but if the last date of a calculation is 31/12/08
then i have a problem as the dates 29/12/08 to 31/12/2008 is based on 2009 rates
i would like to manipulate the print screen using a formula to do the following

if the to date is between 29/12/08 to 31/12/2008 then apply rate as if it was 2009
which will get them at the right rate
i was thinking of something like =if(c1=39813,c1=39814,39810)

which is wrong i suppose
or should i use as in row 1 the total(f1) divided by number of days(d1) less 3 2 or 1 days multiplied by rate on row 1 e1 plus rate on row 2 divided by 6 and add together? so
(f1/d1)-3*(e1/6) + (f2/d2)+3*(e2/6)

any help thanks
r