I'm not sure about the other space agencies worldwide, but I'm speaking with regard to my observations of ISRO satellite launches. read more
Also, like the NASA staff above mentioned, the actual rocket countdown happens with at least more than a minute or half an hour or even more on actual video of a rocket launch. If, we do somehow count up, then we will need to set certain number to be the bar, and the traditional system to see zero to countdown disappear would create much inconvenience to set a new standard. read more
If you launch the rocket at Time = 0, then anything before you launch is a negative number. Twenty seconds before you launch would be -20 on the graph. At least, that's how I think about it. read more
Actually, you could launch a rocket to a piece of music, but somehow engineers prefer to count down boring numbers instead. Imagine how dancers train for e.g. a huge ballet performance. Launch and ground controllers train equally along all those numbers. read more