I am trying to sum a number field (days) that first looks up a date field (Date From) in the current form to see if it is greater than another date field (dateToVacSick) in a related form (vd) but I'm having an issue with dataease accepting the derivation formula. The error message is "Error 513 Setting Derivation Formula. Missing form or files in lookup." Any clues?

days field derivation formula:

sum of "vd" (with "Date From" >= "dateToVacSick" ) "days"

You can't use a With statement in a derivation on a field.

with is a ad-hoc relational statement that only works in DQL.

In DE8 you could simply run a ExecDQL statment that did this an posted the result in a field, but in 6.x you are limited to the relational restraints of a predefined relationship.

A predefined relationship is limited to equal i.e. this field = that field and this field = another field etc.