Edit: Dupe...guess I should check for replies between mashing the preview button.

"My business days are Sunday to Thursday"

Funny, that example is specifically mentioned in the Date::Manip::Config docs:

WorkWeekBeg must come before WorkWeekEnd numerically so there is no way to handle a work week of Sunday to Thursday using these variables...However...can be handled using a fairly simple workaround. To handle a work week of Sunday to Thursday, just set WorkWeekBeg=1 and WorkWeekEnd=7 and defined a holiday that occurs every Friday and Saturday.

It suggests using Date::Manip::Recur for the Friday/Saturday holidays.