That's because:
1. There is a policy on the forum in which we would like to see genuine effort on your side(code written) to help you with a specific question and not make for you the whole app or write you a detailed tutorial just for that task, regardless if its simple or complex. Whether homework or not.
2. For the program you're trying to build, numerous examples are available online for a count down timer in Java. A key class that can be very useful is
javax.swing.Timer
3. The IDE is not that important to be mentioned. The same program can be built with any other IDE or no IDE, while the same logic is followed in terms of code. The only benefit Netbeans for your case is the GUI editor where you don't have to write any code for your Swing based app. And using the Nebeans WYSIWYG editor is trivial(again lots of tutorials and videos out there just for that).
4. What you're trying to do is a simple program that anyone who knows how to program and knows Java can do within 30 minutes or less. Even if you don't know Java if you Google a bit and check samples you can hack something out that works. If you are not a programmer then learn how to program the right way and stay away from the IDE until you're good with the basics. If you think this is a hard task yet consider yourself a programmer, do yourself a favor and quit all this type of work (No offense intended).
That's why, no one would answer such a thread. Because no one is going to build it for you or write you a full tutorial just for that. If this same question was asked somewhere like stackoverlow it would have been down-voted and removed almost immediately.
Have a good day.