thanks for the tips, i found a way to do it:
created following formulas:
{@previous month } :
month(datefield X)=month(dateadd('m',-1,currentdate))
{@currentmonth}:
month(datefield X)=month(currentdate)
{@selection period}:
if {@previous month }=true and day(datefield X)>=25
then 'SELECT'
else if {@currentmonth}=true and day(datefield X)<=24
then 'SELECT'
and then use formula {@selection period} in the select expert.
and for the performance of the report,(quick running and retrieving data)
just putted in the select expert (datefield X) >=today-40
otherwise if your files contains millions of records,every record will be read
by crystal and later selected by the formula {@selection period}.
regards
Memoli