Changing a time series from daily to monthly.

[RKP]

Hi guys,

I have a problem i need to solve during this saturday. I have transfered a time-serie from data stream containing daily change in the S&P 500 index from today and back to 1985. A total of over 6500 daily changes.

What is my problem?

My supevisior want me to change the data-serie from daily to monthly.
Example, 1.jan-31.jan = ONLY JAN

Which means I need to do something, but I dont know exactly how.
I tried to use pivot table, but I didnt get it right.
If any one understood my problem and might have a solution I would be very thankful!
I have attached the file, if that might help you to understand-

Have a nice day, folks.