Earth T-Rex clock not keeping time
There is a battery inside the display unit that powers the clock. It needs to be this way so that it would not reset if the main battery is removed and also to avoid draining the main battery below safe levels if the bike is stored for a longer duration.
When the clock reset's it's because the internal display battery needs charging.
It charges slowly whenever the display is turned on. Normally it's enough that it stays topped up just from the charging when riding the bike normally.
In this case when it's gone low and riding isn't enough to keep it topped up it needs one good long charge.
First turn the automatic shut down timer off - double tap the M key quickly to enter the menu and set the timer to never.
Now that you have the automatic shut down timer turned off you can leave the display on (with the bike in power mode 0) overnight. This can be done with or without the main battery charger connected. It only uses less than 0.5% of the main battery.
After this the battery will be charged and you can set the time and it will hold.