Open the report in report writer
At tool box select calculated field - click new
At calculated filed defination window
1) Enter name of calculated filed - ExpiresOn
2) Change result type to Date
3) select sales transaction work at resources drop down
4) select document date at field
5) hit add
6) click on "+" operator
6) click on constants tab
7) ensure the type is integer
8) enter 90 at constant
9) hit enter for constant
10) hit add
11) click ok
Now the ExpiresOn field will appear at your calculated fileds which you can use in the report.