Hi Brian, 

This is a little out of the expertise of the Workbooks Support Team, however from oding a bit of research I do not think that this is possible in the version of XSL that Workbooks uses for pdfs. 

I would reccommend doing all of the relevant calculations in a Workbooks Report beforehand, and then auto populating a date Field on the invoice with the use of a Dynamic Linked Item. Your pdf code can then call directly on the Field in Workbooks that has had the calculations performed already. 

Best Regards,

Workbooks Support