I'll take you through the hour portion. the coding is all pretty much the same for each bit after that (ie; seconds, minutes, day, etc.) If you need more detail there's plenty of good tutorials out there. just search for flash clock.
ok. so first you need to set up a variable for the hours;
then you need an event listener to run that will update the time. I placed it on a movie clip that's always there like the back of the clock with the numbers on it;
so the currentDate and .getHours(); pull the time from the computer that's running the flash (which is why it's the current time for everyone who looks at it) and the bit of math there turns the movie clip that is the hour hand of the clock to the correct position.