We created Planned Production order then used function Key and selected Refresh then option with all lines - backword. It throws the error message “Error when caluculating backword. Calender is not available before 01-01-98 for work center TKWC1007” even though starting date is 21-01-2002 and ending date is 31-03-2002 which also confirm with routing period calculation considering all the levels, hence asking for a calender 3-4 years old is amusing and not logical. kindly sujjest solution or how to go about it.