It's more than just figuring out the time, not all skills actually trigger global. Sometimes they do trigger it, but not for OTHER skills. In the past, though, patch notes stated that global cooldown was set to 1s. I honestly think it's less than that, something like 0.5 to 0.8s, but the official word unless it was changed somewhere along the line was a 1s global.
[Edit]: Furthermore, if a cooldown lasts longer than global, I think it uses that cooldown instead of the global one. I could be wrong there, but I haven't really bothered testing it to be absolutely sure. A 1s difference can be mostly ignored simply because you can never be 100% perfect with your timing all the time. You have to include some leeway for human response time and reaction to what mobs do.