Fill handle increments year instead of day

I'm trying to use the fill handle to fill a series of dates one day at a
time. The cells are formatted as dates, and the first few are filled like
this:
May-17-2010
May-18-2010
May-19-2010
If I select the first cell and drag the fill handle down, I get:
May-17-2010
May-17-2011
May-17-2012
If I select the first three cells and drag the fill handle down, I get:
May-17-2010
May-18-2010
May-19-2010
May-17-2011
May-18-2011
May-19-2011
What I want is to get this:
May-17-2010
May-18-2010
May-19-2010
May-20-2010
May-21-2010
May-22-2010
I've tried dragging the fill handle with the right mouse button, but the
menu options that appear for "Fill Days", "Fill Weekdays", "Fill Months", and
"Fill Years" are all grayed out.
This is Excel 2010, in case that matters. How can I make this work?
Thanks,
Richard Koett.