system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know days were shorter in the past?
gpt
Geological tidal records show the daily/seasonal deposits of silt. Which show that the hours in a day during the reign of the dinosaurs was about 21 hours

The moon is the cause of the elongated days. Slowly slowing the Earth down.

Edit: Just to clarify, the tides aren't the only contributor to the slow down of rotation of the Earth.  Angular momentum and the tides contribute. 

The main relation the tides have in regards to OPs question is that they have left behind evidence.