how to construct sql

I am grabbing a printer total and putting it in a table.  The
page_count is continuously increasing:

page_count_count        page_count_pdate
10                               2010-05-10
20                               2010-05-10
40                               2010-05-11
60                               2010-05-11
80                               2010-05-11
100                               2010-05-12
120                               2010-05-12
.....

and so on.

I can do:

select sum(page_count_count) from page_count group by page_count_pdate.

and get a total for a day.  But this is not the total I want.  I want
the total page count for the day.  This would mean getting the first
page count of the day and then subtracting that from last page_count
for the day.  For 2010-05-11 above it would be

80 - 40 = 40 total for the day.  Is there a way to do this with sql?

thx,

-wes