What does Uptime mean exactly

I’ve done some searching and I’m sure this has been answered, but I cannot find it anywhere. I pretty much want to know what its the uptime of, does it cover the entire fight? And if so how do we get fights with things like Power Surge, 10s Duration 60s cooldown, being up 70% of the fight. Or does it just cover the possible, like Power Surge could have been up for 70 seconds in this fight, it was only up for 50 seconds, so boom 70% uptime?

Thanks in advance!

Uptimes are wrong a lot of the time because of bugs in ACT not recording buff fades.