As titled, i just want to add a label showing how many new Ads were posted within the last 24 hours. Could anybody let me know how to do that?

 you can do one of two things,

You can either insert a line into a database witha  time and date stamp and select count * from XXX where Datetime.hours < 24 or something like that.

You could open up an file you keep on the server, keep 1 digit in there and add one each time a add is displayed and pull from that number when the label shows the number.

I would suggest SQL server route if you have that ability. 

Let me know if you need help. 

Thanks for your quick reply. I am totally a newbie to .net. So i have no idea how to achieve what you said. Could you kindly provide more detailed instruction? Thank you very much.