$duration($calc($uptime(system) / 1000))

Or if you want to split it up:

var %uptime = $duration($calc($uptime(system) / 1000))
var %secs = $gettok(%uptime,-1,32)
var %mins = $gettok(%uptime,-2,32)
var %hours = $gettok(%uptime,-3,32)
;etc

$input(Me like stars, You too?)