calculate number of days
"Maybe linked to a date of 31/01/09? and then when i change the date
to feb (28/02/09) the number of feb days are shown?"
Given you have the last day of the month in question in D1 then you
can adapt Lars' suggestion as follows:
=MAX(0,MIN(C2,D$1)-MAX(B2,D$1-DAY(D$1)))
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