It is best to use pool.ntp.org as an NTP server. This will find the closest available servers for you.

Also

• pool.ntp.org
• 0.pool.ntp.org
• 1.pool.ntp.org
• 2.pool.ntp.org
• 3.pool.ntp.org

More specific are:

• ntp0.ja.net (Stratum 1)
• ntp1.npl.co.uk (Stratum 2)
• time.cloudflare.com (Stratum 3)

• uk.pool.ntp.org
• 0.uk.pool.ntp.org
• 1.uk.pool.ntp.org
• 2.uk.pool.ntp.org
• 3.uk.pool.ntp.org

• europe.pool.ntp.org
• 0.europe.pool.ntp.org
• 1.europe.pool.ntp.org
• 2.europe.pool.ntp.org
• 3.europe.pool.ntp.org

The 0, 1, 2 and 3.pool.ntp.org names point to a random set of servers that will change every hour.

You can test NTP manually using Windows using the following command (where 192.168.1.1 is the IP of the NTP server).

w32tm /stripchart /dataonly /computer:192.168.1.1

Resynchronize the clock

w32tm /resync

Show current timezone settings

w32tm /tz

List external NTP servers (peers)

w32tm /query /peers

REMEMBER It is important to note that W32Time will only actively synchronize with one time source at a time, even though you are able to list more than one time source. (source)

Most NTP implementations would poll all the servers to choose the best sample, but as of Server 2008, Windows doesn't. That may have changed since then, but it isn't very documented.

w32tm /config /manualpeerlist:0.nl.pool.ntp.org,1.nl.pool.ntp.org,2.nl.pool.ntp.org,3.nl.pool.ntp.org,0x1 /syncfromflags:MANUAL /reliable:yes /update ; net stop w32time ; net start w32time ; w32tm /query /peers /verbose

w32tm /config /manualpeerlist:time.cloudflare.com,0x1 /syncfromflags:MANUAL /reliable:yes /update ; net stop w32time ; net start w32time ; w32tm /query /peers /verbose

sudo date -s "22 JUN 2020 13:11:30"

Check status of NTP on Ubuntu 16.04+

timedatectl status

If NTP is not on, try the following command

sudo timedatectl set-ntp on

List of Stratum 1 servers

• NIST
• ntp.org

National Physics Laboratory hosts two public “Stratum 2” servers. This means their servers sync directly to atomic clocks.

DO NOT configure your system to use Google Public NTP together with non-leap-smearing NTP servers. e.g. do not combine Google NTP with Cloudflare NTP as they do not implement leap smearing. Smearing is used by Google, AWS, and Facebook NTP servers but not Cloudflare.

More information on Google's NTP offering can be found here. Google 'smear' a leap second over the course of a the hour either side of the leap second. Do not mix NTP servers that 'smear' and NTP servers that do not 'smear'.

Google uses its own NTP servers (see above) for its public DNS servers (see below). If you cannot configure multiple servers, time.google.com will automatically select an appropriate Google server nearby.

• IANA's page on timezones
• List of Timezones
• List of Timezone Abbreviations

Using the burst option against a public server is considered abuse. Do not use this option with public NTP servers. Use it only for applications within your own organization.

Burst and iBurst are only relevant if the NTP Servers you are synchronising to actually support these features, if not they can be switched off.

The 'burst' option is used to increase the average quality of time offset statistics. At every poll interval, when the NTP server responds, the system will send a burst of up to eight packets instead of the usual one packet. For use with the server command to improve the average quality of the time-offset calculations as well as accurately measure jitter with long poll intervals.

The 'iburst' option is used to improve the time taken for initial synchronization. When the NTP server is unreachable, the NTP client sends a burst of eight packets instead of the usual one packet and then shorten the time until the first sync. The packet spacing is normally 2 seconds. The iburst mode continues to send frequent NTP queries until the server responds and time synchronization starts. After the first minute, the iburst mode typically synchronizes the clock so that queries need to be sent at intervals of 64 seconds or more.

Microsoft only support Simple Network Time Protocol (sntp) which is compatible with NTP clients but does not offer all the functionality of NTP (including Burst and iBurst). Microsoft servers will just ignore the burst and iburst options.