SubBusinessDay function??

SubBusinessDay function??

Hi 

I wonder why there is no SubBusinesDay date function? or alternatively why we are unable to use minus days with the addBusinessDay function?

In my app I have a deadline date that is fixed and I have a number of business lead days necessary to calculate the date that a project needs to start by to meet the deadline date. While I can use subDay, it doesn't take into account non-business days.

Any ideas of how I can accomplish this without this date function?

Thanks!