Sum of previous offset number of cells not to exceed certain value
 

I am creating timesheets for workers by using a worksheet for each
month of the year. The Worksheet cells represent 30 minute units of
time and they enter a "W" if working.
Each day of the month is on a separate Row.

The rules require workers not to work more than X hours in any 24 hour
period. I need to create a "rolling" 24 hour total to check this rule
but I am stuck.

So each 24 hour period includes nentries from the previous Row.

A further complication is during the first day of the month, I need to
include hours from the last day of the previous month (on previous
worksheet).

Any suggestions or help would be greatly appreciated.

Simon Morgan