XNPV function with starting point dependant on date

[Carmen]

I asked a similar question for the "sum" function, but the answer I got for it will not work for the "XNPV" function, which is what I really wanted. So I will ask it here (again, sort of). I have cells A1 through B4

1/1/06 50
2/1/06 40
3/1/06 20
4/1/06 30

I want to calculate

XNPV(10%, A1:A4, B1:B4), XNPV(10%, A2:A4, B2:B4), XNPV(10%, A3:A4, B3:B4),
XNPV(10%, A4:A4, B4:B4),

with the starting point B? dependant on the date I enter in another cell C1 (the date
in C1 would of course correspond to a date in B1 to B4), some kind of
"lookup" function embedded in the XNPV function?

Thanks