The following link is to a draft of a Redbook for DB2 Web Query. Chapter 12 has info on data manipulation.
In addition, youu can manipulate dates in the Report Assistant using a computed field. Create a new computed field and in the box where the calculation is needed put something like this:
JULIAN/I5 = JULDAT(<i>YOUR_DATE_FIELD</i>, JULIAN);
GREG_DATE/I8 = GREGDT(JULIAN, ‘I8′);