<< When batteries are removedm, the backup cell is able to sustain
the data for some 20 hours. >>
I've never measured it formally, but that sounds about right to me.
It's the reason why I recommend alkaline batteries as your main batteries.
Lithium batteries drop off suddenly, and when they do, the unit relies on
the backup. If you put the unit away for a weekend and the mains are
lithiums nearing the end of their lives, they could drop off on Saturday
and the backup might not last till Monday.
If you are using alkalines, however, they retain enough charge, even when
weak, to avoid draining the backup for many days.
Steve