PROBLEM SOLVED
At startup :
if flag 'power_lost' is True:
the external DS3231 is set with pre-determined datetime (adjust by hand)
next the built-in RTC is started with the datetime from the DS3231
if the flag 'power_lost is False:
the built-in RTC is started with the datetime from the DS3231
During the main loop displaying the datetime from the built-in RTC.
I put a short video in the images folder inside the dropbox folder (see the link in my post above).