The Bafang C860 display found on the first generation (black colored) Atlas has an internal battery in it that should keep power to the clock while the bike is powered off. If the bike is not ridden frequently enough or long enough the battery could deplete, which causes the clock to reset.
The resolution to this is to keep the display powered on for 72 hours, which then allows the battery in the display to charge up from the main battery on the bike. In order to do this, turn off the auto time off function in the display settings so that you can leave the bike sitting in the garage/storage with the display powered on. Make sure the charger is not plugged in otherwise the bike will not power on; the main battery will not deplete over the course of the 72 hours, but its a good idea to periodically check the bike.
See information below from the display manual.
