I've actually never used the lookup function, but in the case your talking about I'd probably use an ApplyMap Function.
1. Create a Map table
MAPPING LOAD Date([DateTran]) as DateKey, San_Mth_Yr FROM U:\...... (your excel file);
2. In your main table load statement replace the lookup function with
ApplyMap('CAL_MAP',invoiceDate,0) AS dd
That should do it.