2021 will be the shortest year in decades. 2021 is set to be shortest year in decades according to experts at Time and Date. The next 12 months will be shorter than a normal year, due to the fact that the Earth is moving faster than it ever has in the last 50 years. The speed of the Earth’s rotation can be affected by a number of factors, including things such as the motion of its molten core, oceans and atmosphere. ‘2021 will be the shortest year in decades’ Due to this increase in rotation speed, the average day in 2021 is expected to be 0.05 milliseconds shorter than the 86,400 seconds that normally make up the 24 hour period, according to Graham Jones, astrophysicist and science communicator, and Konstantin Bikos, editorial team lead at Time and Date.