I've run into a problem when using autofill in Excel. I'm trying to autofill
dates in the following format dd/mm/yyyy h:mm:ss. For example, I have a
measurement time of

02/10/2008 12:28:00
and the next time is:

02/10/2008 12:28:15

I've highlighted both of these cells and then dragged the bottom corner to
autofill this pattern. This works for awhile until I get:

02/10/2008 12:40:29
instead of
02/10/2008 12:40:30

This doesn't make a big difference right away but after continuing this for
a couple of hours the time is way off.

Is Excel all of a sudden rounding this number? Is there a way of making it
so that exactly 15 seconds are added each time? Any help would be greatly
appreciated!