Microsoft project formula again

[smtamblyn]

My project starts on the 5th january 2009, using our own calendar, therfore day 1 is the 5th jan, and to use a base line, day 75 should be 22nd april 2009. I have customised the 'duration10' field, which has been converted from a number field to show days, so that the number 75 in the number field also shows as 75days in duration 10. I have then used a customised start field and tried 2 formulas to work out what day 75 should be, formulas tried were:

Project start date + Number 20 ( date returned was the 21st march )

Project start date + Duration 10 ( errors returned )

Help ???!!!!

[Lemog]

The [Deliverable Start] and [Deliverable Finish] fields are available fields in the Custom Fields dialog. It's just that they don't show up as available Fields when you click the Field pick list in the Formula dialog. I created a custom Duration field for the Task type, named it Deliverable Start Variance, and manually typed the entire following formula:

IIf([Deliverable Start] = ProjDateValue("NA"), 0, ProjDateDiff([Deliverable
Start], [Start], "Standard"))