No, it would almost work like $nick(#,nickname).idle, but it would be different. If you join the channel, and everyone is there, everyone is essentially set to zero, and grows. When someone leaves and rejoins, it's reset to zero. It would be done for ON JOIN to reset it to zero.
But if that's not done, I would think it's time to add a built-in creation time for windows so $window(window).ctime.
