it is entirely client side, it wont change anything on the server.
it works by spoofing the internal clock = telling your minecraft that time is going faster than irl
it looks for other players like u being in a timelapse.
PS: it is also possible to slow down your minecraft (useful on laggy servers with trash-anticheat that rubberbands you, if ur not also lagging) This is done by setting the timer value below 1
PS2: it is also possible serverside, but u have to install the carpet mod on the server. its called tick warp.