How to build a business process which start every time at a defined time, such as every 8:00 am per day or some time in a day?.

Here is an example of a simple business process,  to run the process in a defined time without send an Input Event. This simple process just comprise one Timer Start Event and one None End Event. And with a sequence flow to connect them.
wirte code in Timer Start Event properties, like this:
TimerManagerRW timerManagerRW = new TimerManagerRW(XPMLConstants.TIMER_INSTANT);
//TODO : insert your Java code here
timerManagerRW.setTimeInstant(2007,10,25,18,5,0);

return timerManagerRW;

After sending the BP to production, please log in ProcessManager Administrator Tool and launch the BP with command start bpi.
Can see the syntax of this command, by typing "help" on ProcessManager Administrator Tool command line.
The BP will be executed when the set time is reached.