Free tool
Best time to upload on YouTube
This looks up the UTC times a channel's recent public videos went live. It does not know when that channel's viewers are online. YouTube Studio does.
These are the UTC times your public videos were published. They are not a measurement of when viewers are online. For that, use YouTube Studio → Analytics → Audience.
Where the real audience hours are
YouTube Studio → Analytics → Audience → When your viewers are on YouTube. That chart is private. A public tool that claims a single best hour for every niche is guessing. The guide walks through how to use the Studio report and then keep a weekday you can actually hit.