It is not very easy to find a sample of how to define a site column of type calculated using the SharePoint 2007 features framework. The documentations shows how to do it using the user interface (see Calculate data in lists or libraries ). The documentation in the WSS SDK about the Formula Element in the CAML Core Schemas points in the right direction but is maybe not clear enough. I tested the sample code below by using the site columns in a list definition. You might want to test it out in a content type as well - please provide feedback in the comments.
Code sample feature.xml
<?xml version="1.0" encoding="utf-8" ?>
<Feature xmlns="http://schemas.microsoft.com/sharepoint/" Scope="Site" Title="Calculated Sitecolumn" Id="49C90DCA-6C76-4b30-99DB-7C4C7D100761">
<ElementManifests>
<ElementManifest Location="calcsitecolumn.xml"/>
</ElementManifests>
</Feature>
Code sample calcsitecolumn.xml
<?xml version="1.0" encoding="utf-8" ?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<Field Name="CalcDemo" StaticName="CalcDemo" Type="Calculated" DisplayName="Calculated column" ID="{DD01AF0E-5E58-4e50-B3E8-1BFC866CA9A0}" ResultType="DateTime" Format="DateOnly" ReadOnly="TRUE">
<Formula>=StartDate+7</Formula>
<Default></Default>
<FieldRefs>
<FieldRef Name="StartDate" ID="{64cd368d-2f95-4bfc-a1f9-8d4324ecb007}" />
</FieldRefs>
</Field>
</Elements>
Have fun with it ...