In my 4 decades of being a supplier on the electrical side in construction, plus ditto to utilities, OEM's, city/state/federal gov't, industrial users and so on, you are correct sman.

Calender days can't be anything else but such.

An RFQ that said "your proposal must be received, time and date stamped in 30 calender days from X date to be accepted for consideration or it is void" is pretty darn clear.

Business days would be M - F excluding Sat, Sun and National Holidays.

No brainer here...