I am trying to find a formula to find the datediff function of two summary values which lies in two seperate groups.

 all advices are most welcomes since i am desperate to figure out this!

I am using the sql server 2008 database for our access control system and depends on the card holder transaction for time attendance. I summarise the transactions (which minimum as the entry time and maximum as the exit time for that particular employee) grouped by date and i use the summaries as group footers and the datediff function to find the time he spends in the company.

the problem is with the night shift employees, where i need the datediff between groupfooter for today(as example) and group footer for yesterday.