Calculate minutes played by players in sport

Please see the attached workbook.

sheet 1 is an event sheet.
column A are the dates of matches. there are 3 matches in total played on the 17th, 24th, and 30th Aug 2013.
in column B there is the player's number involved in the event.
column C is the event.
the words that need attention on column C are "Sub Off" and "Sub On".

I would like sheet 2, column C to calculate how many minutes the player in column B played on the date listed in column A.

remember the match lasts 80 minutes. thats the maximum a player could have played.
so if a player wasnt "sub off" then they obviously played 80 minutes.
if a player was "sub off" then they played for whatever column D says...
and the player that replaced them on "sub on" played for 80 minus whatever column D says .

hope that all makes sense

thanks