Access a youtube playlist:
https://youtube.com/playlist?list=PLMC9KNkIncKtPzgY-5rmhvj7fax8fdxoj
Open the developer console using F12
Paste the code below.
let date = new Date(null);
document
.querySelectorAll("ytd-thumbnail-overlay-time-status-renderer")
.forEach(video =>
date.setSeconds(
video.innerText
.split(":")
.reduce((accumulator, time) => 60 * accumulator + +time)
)
);
console.log(date.toISOString().substr(11, 8));