How to calculate formulas on my own?

I have some example of data which are daily values.

At the end of each month it should be calculated percentage increase/decrease compared to one month later. Is this possible with excel or only manually calculation? Months do not have the same number of days... Need help

It's difficult to be definitive about such a vauge question...
However, look at the date related functions in the excel help file,
type in Date to the help search then look at the various topics supplied.
Follow the See Also links.

Without much effort you should be able to devise a formula to give the number of days in a given month.

try =DATEVALUE("1/3/2008")-DATEVALUE("1/2/2008")

Mark.

You will likely get a better response if you post a workbook example.