copied info for chart does not show date correctly

I have Excel 2003
I have a chart that I copy over from month to month. It shows the date that
data was turned in from several different units each month. the problem is
that although I enter the correct date into the excel sheet, the date from
the last month shows up on some of the units. but not on all! that is what
seems so odd.
I checked to see the correct dates were entered and checked the
source/series to see it was referring to the column with the dates I entered.
but on 4 of the units (there are 11 units in total) the date is from the
month before....
Any ideas?
Thanks, Meenie