"An ‘Earth Day’ is 24 hours long, and that more precisely it is 23 hours 56 minutes and 4 seconds long. But this hasn’t always been the case. Detailed studies of fossil shells, and the banded deposits in certain sandstones, reveal a much different length of day in past eras! These bands in sedimentation and shell-growth follow the lunar month and have individual bands representing the number of days in a lunar month. By counting the number of bands, geologists can work out the number of days in a year, and from this the number of hours in a day when the shell was grown, or the deposits put down." Credits: NASA
