Here is a user-defined function that will do it. 'dt3' is the date for
which you want a value that falls between val1 and val2.

Function Interpolate(dt3 As Date, dt1 As Date, dt2 As Date, _
val1 As Double, val2 As Double) As Double
Interp = (dt2 - dt3) / (dt2 - dt1) * val1 + (dt3 - dt1) / (dt2 -
dt1) * val2
End Function

Hth,
Merjet