XIRR function input parameters
 

I use the XIRR function regularly but would like to input relative referneces
instead of always using arrays or constants (as in the Help example) but if I
try this I get an error.
Specifically I would like to do something like this: xirr({-a1,a2},{b1,b2}).
This way I can calculate the period return from a list of positive values
(e.g. balances as of a certain date).
Thanks,

Kieran