Calculate average weekly, monthly, quarterly and annually?

Hi,

I am doing a Twitter analysis and I have the data in .csv format. In that I have date_published and sentiment_score two column along with some columns. But these two are now important for my average extraction.
I have to find out the average of the sentiment_score column ie from 2007 to 2011 weekly, monthly, quarterly and annually. The problem here is the dates are irregular, i.e consider first week of January 2007. 01/01/2007 to 07/01/2007 but it has 03/01/2007. And for some weeks one day is missing for few 3 days are missing and for few weeks on the same day there are 4 tweets. So how can I calculate the average for this. I posted an example and attached a sample data set. I am really stuck at this point as doing manually will take a lot of time at least for weekly and monthly for 5 years long data. If any one knows any solution or can help me it will be really helpful.


Example posted below:
Date_published
03-01-07
12-01-07
15-01-07
15-01-07
16-01-07
16-01-07
15-01-07
17-01-07
17-01-07
17-01-07
18-01-07
19-01-07
20-01-07
22-01-07
22-01-07
26-01-07
27-01-07
29-01-07
31-01-07
31-01-07
31-01-07
30-01-07
02-02-07
02-02-07
02-02-07
02-02-07
02-02-07
02-02-07
02-02-07
03-02-07

Thanks,

Karthik