hello everyone. just got a problem at work... i have a list of dates with events on those dates.

i have to make a situation depending of each date. like subtotala on every day. porblem is that the dates are in fact intervals of 12h.

so the 12h i have on the day time is written like: 31-01.04.2011 (meaning the night between 31.03.2011 and 01.04.2011).. or 31.04.2011 its the day.. so ..

i have a list like:

date no of trucks
30.03.2011 12
30-31.03.2011 10
31.03.2011 1
31.03.2011 3
31-01.04.2011 1
20.03.2011 3
11-12.06.2011 10

and so on.. how do i make subtotals based on the 12h shifts.. without changing the format of the date AND the dates in the subtotals to be sorted from small to big..

can anyone help?
thanks