I have a similar situation (fiscal year instead of tax year) and I do this in queries or on forms/reports with a formula like this:
Code:
IIf(CInt(DatePart("m",[transactiondate]))=4,(IIf(CInt(DatePart("d",[transactiondate]))>=6,DatePart("yyyy",[transactiondate])+1,DatePart("yyyy",[transactiondate]))),IIf(CInt(DatePart("m",[transactiondate]))>=5,DatePart("yyyy",[transactiondate])+1,DatePart("yyyy",[transactiondate])))
Then you don't need to do any sort of lookup or any data conversion you just add this calculation as a field in your query and process only the tax year you want.