How long is Earth's day? We now have the most precise answer to date 

 A laser gyroscope at an underground lab in Germany has detected the minute variations in Earth's rotation, an advance that may help scientists build better climate models. 

https://cdn.mos.cms.futurecdn.net/rUrk9y2Qu4722JagVpmLcm.jpeg

https://www.space.com/earth-day-length-measured-unprecedented-precision