1. About the exact time
Today, the issue of ensuring uniform time has become particularly relevant. The uniform time is needed in the work of transport, communication, energy, various financial institutions and banks.
Due to the widespread use of computer networks, the problem of achieving the uniform time in all network devices has arisen. Correct data processing in real-time systems is impossible without time synchronization.
In information protection systems, linking a message to a specific time stamp allows protecting the message from being changed during transmission over the network.
To solve these problems, the system time scale of network computers must be synchronized with the UTC time scale. The system time scale of most computers, due to the properties of clock quartz generators, does not differ in high stability. This causes the need for constant correction of the system time scale.
Modern computer technology provides the following opportunity: synchronization signals are transmitted using standard computer communication equipment.

2. Technologies for synchronizing computer timelines.
2.1. Using telephone channels for time synchronization.
Data is provided in ASCII code. As a rule, the transmitted data contains the modified Julian date, the current date (number, month, year), current time (hours, minutes, seconds), summer (winter) time indication, indication of the need to add (subtract) a leap second on the last day of the current month, and other official information. The last character of the string to be transmitted is the synchronization character. The moment of arrival of this symbol on the remote computer is the moment of arrival of the second whose code was contained in the transmitted parcel of information. The time code transmission delay is either fixed or measured multiple times during the messaging process. This delay is compensated. This method of synchronization has been used for a long time by metrological services in a few NMI: NIST (USA), NPL (Great Britain), PTB (Germany), etc.
2.2. Daytime protocol
Next, Internet time transfer protocols are considered. According to the Daytime protocol (RFC-867), the current date and time are represented by a string of ASCII characters. TCP and UDP transport layer protocols are used for transmission. There is no hard limit on the format of the transmitted string. The recommended format is day of the week, month, number, current time, time zone.
2.3. Time protocol
According to the Time protocol (RFC-868), the current date and time are represented as a 32-bit integer specifying the number of seconds since midnight on January 1, 1900, and UDP.
The Time server waits for a connection on port 37. As soon as the connection is established, the server sends a 32-bit binary code of the current date and time and closes the connection. Similarly, when receiving a UDP datagram, the server sends a response datagram containing the above data.
2.4. Sending ICMP Timestamps
The ICMP protocol (RFC-792) is intended for the transmission of service messages during various control and test operations in the network. Among the messages provided by the protocol format are messages containing time stamps (Timestamp Message and Timestamp
Reply Message). The exchange of these messages is necessary, for example, to analyse the time of traffic in the network.
2.5. NTP protocol
The NTP protocol (RFC 1305) provides for the transmission of UDP messages containing 64-bit timestamps with an accuracy of about 200 picoseconds. Special algorithms for correcting errors related to network transmission delays and deviation of the frequency of the computer's crystal oscillator from the nominal value, as well as algorithms for selecting a time server with better characteristics, ensure the highest accuracy of time unit transmission compared to the protocols described above.
The SNTP protocol is a simplification of the NTP protocol and is used in case the second accuracy of synchronization is sufficient. The timestamp format of this protocol matches the NTP timestamp format.

3. Our service.
As for today, open access is provided to two stratum 1 level servers, which are synchronized with the time scale of the State primary measurement standard of time UTC and frequency of Ukraine (UA):
- ntp.metrology.kharkov.ua (Symmetricom SyncServer S250i);
- ntp1.metrology.kharkov.ua (FreeBSD).

4. Service settings.
4.1. Connecting to the time server using the Internet. To use the service of connecting to the time server using the Internet, the following hardware and software are needed: - IBM-compatible computer with a processor of at least Pentium 166 and RAM of at least 16 MB
- Internet connection
- Windows 9x/ME/NT/2000/XP/2003 operating system or UNIX-like system with NTP protocol support (RFC 1305)
If you want to use the service in the environment of a UNIX-like system, the following actions must be performed:
1. The ntpdate program from the NTP package <link to the source texts of the package> is used to obtain information about the time on request and to correct the computer's time scale once. To run it, the following command must be executed:
ntpdate ntp.metrology.kharkov.ua
2. It is recommended to use the ntpd or xntpd programs, which are also part of the NTP package <link to the source texts of the package>, to periodically obtain information about the time and correct the computer's system time scale. To work correctly with the time server of the NSC "Institute of Metrology", the following entry must be added to the configuration file /etc/ntp.conf:
server ntp.metrology.kharkov.ua.
3. Work according to the Time protocol is carried out using the rdate program. To synchronize the time with the server of the NSC "Institute of Metrology", it is necessary to execute the following command:
rdate -s ntp.metrology.kharkov.ua.
4. The TSP protocol is implemented by the timed program, which also allows periodical adjustment of the system time scale. To synchronize the time with the u1061 server of the NSC "Institute of Metrology", the following command must be executed:
timed -F ntp.metrology.kharkov.ua.
If you want to use the service in a Windows-NT/2000/XP environment, the following actions must be performed:
1. Windows-NT/2000/XP operating systems have a built-in ability to synchronize time using the NTP protocol, implemented by the command:
NET TIME/SETSNTP:ntp.metrology.kharkov.ua.
2. An alternative option is possible. It is necessary to call the "Properties: Date and Time" applet from the Control Panel and select the "Internet Time" tab
1.1 After that, the checkbox "Perform synchronization with the time server on the Internet" should be activated.
1.2 After that, enter ntp.metrology.kharkov.ua in the address bar "Server" and click the "Update Now" button, then OK. Your computer is now synchronized with the time server of the NSC "Institute of Metrology"
Note: For the continuous synchronization, constant access to the Internet is needed.
If you want to use the service in the Windows-9x/ME environment, the following actions must be performed:
1. Since Windows-95/98 operating systems do not have own software tools that work according to the NTP protocol, so software packages must be installed for time synchronization
NetTime <link to file with package> and Tradis-2000 <link to file with package>, which allow working with other time transfer protocols.
2. To configure the Net Time program, you need to go to the "NetTime
Options" and specify the parameters:
- server address;
- Protocol (SNTP, Time/TCP or Time/UDP);
- port;
- Update Interval (time interval between requests to the server);
- Retry Interval (time interval between additional attempts to access the server in case the server is unavailable);
- Max Free Run (time interval of operation in the absence of synchronization, after which the computer clock
are considered unreliable);
- Warn if Adj Greater (maximum allowable value of time correction).
3. After installing the Tradis-2000 package, the server address and protocol type in the "Server Details" window (enter by clicking the "Add" button) and the time interval and calls to the server (How often time is set) must be set, as well as the " Set the time" ("Set the time") in the tab "Setting the time" ("Setting the time") must be checked.
Note. If your computer is connected to a local network, most likely that the u1072 address ntpd.metrology.kharkov.ua will be unavailable to you, and the computers will be synchronized by the local network server.
Therefore, consult your system administrator for the correct installation of time synchronization programs on your computer.

5. Links.
NTP developer site (University of Delaware)
www.ntp.org
List of time servers on the University of Delaware website
www.eecis.udel.edu/~mills/ntp/servers.html
Web page of the department of time-frequency measurements
National Institute of Standards and Technology of the USA
www.nist.gov/timefreq/
USNO Time Service Web page
tycho.usno.navy.mil/ftime.html
BIPM time server web page
www1.bipm.org/en/scientific/tai/time_server.html
NPL Time Service Web page (Great Britain) dedicated to computer time synchronization
www.npl.co.uk/time/truetime.html
NRC Time Service Web Page (Canada)
inms-ienm.nrc-cnrc.gc.ca/time_services.html
The site of the laboratory, which is engaged in the transfer of the unit of time to PTB
www.ptb.de/en/org/4/_index.htm
Solutions (hardware and software) of the Symmetricom Corporation in the field of synchronization of computer networks
www.ntp-systems.com