This is just my opinion, but I am not a big fan of the default time servers in the ntp config of the Raspbian OS. Something like debian.pool.org or some -ian like word. I stick to the NTP.ORG time servers.
Most of the time the global pool pool.ntp.org (or 0.pool.ntp.org, 1.pool.ntp.org, etc) is sufficient.
The ntp.org system will try to find the closest availible servers for you.
But you can choose one that is closer to your location.