I tried grouping a date field in a pivot table but
only got a "cannot group field" message. This was
because although I reformatted the date without the
time, underlying it was still the time of day which
you could see by clicking on the cell and seeing it
in the formula bar. By moving the date column, in the
list my pivot table was based on, over to the far left
column, the grouping feature had no problem asking if
I wanted the date field grouped by day, month or year.