Count down timer
I'm trying to build a simple count down timer. I found an excellent example in this video: https://www.bing.com/videos/search?q=%22storyline%22+timer&ru=%2fsearch%3fq%3d%2522storyline%2522%2btimer%26FORM%3dHDRSC1&view=detail&mmscn=vwrc&mid=4C06DD850507D6D9FA7C4C06DD850507D6D9FA7C&FORM=WRVORC
This timer adds number, but I already figure it out how to subtract numbers instead of adding numbers. My question is: how can I make the timer stop when it gets to "0". Can anybody help me to solve this problem? It must very easy stuff but I'm blocked. Or is there any other example I can watch? I've checking a lot of videos and reading a lot of information but I still can find the answer. Thanks in advance.