I must add that it seems that the ntp.ubuntu.com server doesn't work at all:
it happened that this morning, the time on my laptop was 10 minutes late
after 1 hour, it was still 10 minutes late
I replaced ntp.ubuntu.com by another ntp server, and 2 minutes later,
the time became correct.
I checked also on another desktop: same result
I must add that it seems that the ntp.ubuntu.com server doesn't work at all:
it happened that this morning, the time on my laptop was 10 minutes late
after 1 hour, it was still 10 minutes late
I replaced ntp.ubuntu.com by another ntp server, and 2 minutes later,
the time became correct.
I checked also on another desktop: same result