Computing monthly standard deviation from daily data

Hi guys,
as the title suggests, I have to compute monthly standard deviation (volatility) from daily data.
The problem consists on the fact that the days during a month are not constant and change over time.
I attach the excel file to allow you to help me.
Price_Time_Series.xlsx
In this xls file, you can find dates in column A, price in column B and returns in column C.
I want to compute standard deviation at the end of the month for stock returns (column C), by considering all availables days.
As you can note from the file, the days change over time and they are note constant.
How can I compute standard deviation of them?
Can you help me, please?
Thanks a lot!

I thought about this, but I think it is not worthwhile, because I've many time series like the example one and I need to find some function or macro to simplify that process.
Anyway, thanks to mind my problem!

Hi Richard,
can you explain me how to use that sheet?
when I open that file, SD is equal to an error as "#NAME?".

no, that's ok. Excel changed automatically comma with the semi-colon;
I think the problem in this case is in the Name Manager, because when I click on SD formula I find:

=_xlfn.STDEV.P(SD_Data)

and I think that on my PC SD_DATA is not defined.

My version is MS Excel 2007.
Anyway I solved replacing STDEVP with DEV.ST.P.
The problem consisted on the fact that Excel didn't convert English formula with the Italian one.
Thanks a lot Richard for the help.
You have been help to me.