TIL the length of a day on Mars has been known to the nearest 3 minutes since 1666. But the exact length of a day on Saturn remained a mystery until 2019.

https://solarsystem.nasa.gov/news/814/scientists-finally-know-what-time-it-is-on-saturn/

https://www.reddit.com/r/todayilearned/comments/151fqnu/til_the_length_of_a_day_on_mars_has_been_known_to/