Report - code builder

Hi to all,

what is the code equivalent for

if ([FieldA] < [StartDate]) then
[FieldB].value=0
end if

[StarDate] is a Date parameter which i entered when the report is created
[FieldA] is a Date field
[FieldB] is a Number field

i want to insert this in the code builder to run before the report is created, so that it changes the results showed

in Design View for a Report u can select to biuld an event an have 3 options:

Macro Builder
Expression
Code Builder

Code Builder looks like VBE and if this would have been Excel i would know where to start from, but being access ...

Here's a .jpg to help explain my problem better

the Report is supposed to show the Total Income for the period entered. My problem is with the Addendum (Act aditional in the report), since it's linked to the Contract and i need to show the contract info for that addendum, but i also have to show contracts info.

The problems appears with addendum for 2012 which have a contract done in 2011, because the info from 2011 appears in the report and affect the sums.

That is why i was trying to resolve this by code.