I want white and shaded alternate rows, but not in the simple one-on one-off
pattern.

My spreadsheet is sorted by date in Column A. I may have 10 rows with a date
of 01/09/2010, then one row with 01/10/2010, then 20 rows with
01/11/2010...you get the idea.

I want all rows with a given date shaded, then all rows for the next date
(and there are gaps of more than one day between some dates) left white, then
all rows for the next date shaded, etc.

The purpose, of course, is to make it easy to distinguish all rows for a
given date at a glance. I have played with conditional formatting for hours
and can't get it right. Would appreciate any help.