# Information on Time

#### What is the use of the standard time equipment?

It serves for the mandatory realisation of the unit of time (second) and the unit of frequency (hertz). These quantities are disseminated to interested parties inside and outside BEV.

#### How are the second and the hertz defined?

The second (s) is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s-1.

The hertz (Hz) is the unit of frequency (number of periodic events per second), where 1 Hz = 1 s-1.

#### How many atomic clocks does BEV operate?

Two industrial atomic clocks with thermal caesium beam tubes and an active hydrogen maser are operated stationarily and, additionally, a battery-buffered rubidium frequency standard serves for mobile use.

#### How does BEV check if the disseminated values for time and frequency are correct?

BEV is constantly carrying out time comparisons with other institutes using GNSS-satellites. About 500 atomic clocks take part in these comparisons. The International Bureau of Weights and Measures (BIPM) at Sevres near Paris calculates a mean time scale (UTC) from the measuring values und informs the institutes about the deviations of their clocks. BEV accounts for these deviations when calibrating if necessary.

#### How large is the deviation of the duration of a second at BEV from the duration of the second at BIPM in Paris?

At the moment the absolute value of the deviation is less than 5 x 10-14, which signifies an uncertainty of less than 1 second in 1 million years.

#### Dissemination of the units of time and frequency by BEV

Time signal and standard frequencies are transmitted through the public telephone network:
Second pulses: +43 1 21110 821505
440 Hz (concert tone A): +43 1 21110 821507
1000 Hz: +43 1 21110 821509

The time signal is transmitted in the form of second pulses with a constant 50 ms tone of 1000 Hz at the beginning of each second. Every new minute is announced by the suppression of the last pulse before this new minute. Furthermore the last 5 pulses before the beginning of the first minute of the next hour are suppressed. These second pulses can be used to set clocks or to calibrate microchronometers (stopwatches) for example.

The deviation of the Austrian Time Scale from the International Time Scale UTC is at all times kept below 100 ns.

The time signal transmitted via telephone line arrives time-delayed at the recipient. This time delay is always below 30 ms for all locations in Austria and her neighbouring countries. The delay has no influence on the determination of time intervals as long as the connection is not interrupted. If the connection was interrupted and restored later on, the new connection could use another path than the old connection.This can result in slightly different delays, whose variations will be irrelevant in most cases. At the calibration of manually operated stopwatches the measuring uncertainties are practically solely caused by manually starting and stopping. Therefore any variations in delays can be neglected. A typical value for the measuring uncertainty caused by manually starting and stopping is 0,3 seconds.

BEV operates three NTP-Servers to set internal computer clocks (NTP stands for Network Time Protocol). The host names of the NTP-Servers are
bevtime1.metrologie.at
bevtime2.metrologie.at
time.metrologie.at
and can be entered as NTP-source into synchronization programs (freeware on Internet). Windows has an integrated client that only requests the input of the address. The synchronization of the PC-clock is mostly better than 50 milliseconds (50 thousandths of seconds).

Additionally, the standard time is also disseminated by a time distribution system via modem. This time distribution system can be reached under the phone number +43 1 21110 826381.

Furthermore, time and frequency are disseminated through calibrations of the Physico-Technical Testing Service. BEV calibrates the most different kinds of measuring instruments for time and frequency, such as stopwatches, electronic microchronometers, frequency measuring instruments, quartz standards, rubidium standards, standard frequency receivers, etc.

The customer receives a calibration certificate quoting all measuring results (measured values, measurement errors, correction values) including measurement uncertainties so that he can consider the deviations of his measuring instruments when measuring.

Time and frequency are also transmitted via internal lines to in-house laboratories.

#### Who needs time and frequency?

The need for time synchronization via Internet has been increasing, recently. Many PCs are used for time-critical long time measurements. For example, sound measurements near airports or railroad lines have to assign all emerging sounds to the right plane or train. The PC can take its time periodically (e.g. daily), so that the error of its internal clock will not accumulate over days and weeks.

Users from the fields of metrology, telecommunication and laboratory techniques have their standards regularly calibrated to make them traceable to the national standards. This proof of the accuracy of the measuring instruments is particularly important in the field of quality control and accreditation.

Since time and frequency are the physical quantities that can be realised with the highest precision, other quantities like length and voltage are traced back to time and frequency. The meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The unit of electrical voltage, the volt, can be traced back to frequency through a quantum-mechanical effect that occurs in semiconductors at the temperature of liquid helium.

#### What are leap seconds for?

In the past the second was not defined by the number of vibrations of a caesium atom, but was derived from the duration of a mean solar day. The old definition would no longer be useful for the measuring accuracy required nowadays since the Earth’s rotation velocity is subject to measurable variations and is also constantly decreasing over the years. Such a second would not be of constant duration, but depend on the Earth’s rotation velocity.

In contrast, the atomic second has a constant duration and is independent of the Earth’s rotation. Sometimes additional seconds, so called leap seconds have to be inserted into the time scale to prevent that the time scales do not surpass Earth’s rotation and to make sure that noon on our clocks remains also the sun’s noon position.

#### Why and when are leap days?

To adjust our calendar year to the tropical year (365.2422 solar days), leap days have to be inserted. For practical reasons, calendar years can only have an integer number of days, namely 365 (regular years) or 366 (leap years).
Whether a year is a regular year or a leap year can be determined by the following rules:

Rule 1:
If the year can be divided by 100 without remainder, but not by 400, then this year is a regular year. According to this rule e.g. the years 1700, 1800, 1900, 2100, 2200, 2300 are regular years. The years 1600, 2000 and 2400 are leap years.

Rule 2:
For those years that are not regular years according to rule 1, applies: If the year can be divided by 4 without remainder, it is a leap year, otherwise it is a regular year. According to this rule e.g. the years 1996, 2000 and 2004 are leap years and 1997, 1998, 1999, 2001, 2002 and 2003 are regular years.

By applying both rules we get 97 leap years in 400 years. The mean duration of a calendar year is 365 97/400 (= 365.2425) days and is therefore close to the duration of the tropical year.

#### When did Austria introduce Summer Time and how are the clock changes scheduled?

Austria applied the Summer time in the following periods setting clocks forward by one hour (Austria never used a “double” Summer time – clocks are set forward by two hours):

bZr:  “bisherige Zeitrechnung” (“previous time calculation”)
NZ:  Normalzeit (Standard Time)
SZ:  Sommerzeit (Daylight Saving Time)
MEZ:  Mitteleuropäische Zeit (CET Central European Time)
MESZ:  Mitteleuropäische Sommerzeit (CEST Central European Summer Time)

(In 1945 Summer Time was suspended without any special decree by the end of the war. In most cases Summer Time ended with the invasion of the occupation troops which was at different times in different regions in Austria; in Vienna it was reportedly on 12 April 1945 (to our knowledge there exists no precise documentation).

Since 1981 Summer time in Austria has been set by decree of the Federal Government for two to three years in advance according to the following rules:

Beginning:  last Sunday in March at 2 h MEZ

End:          until 1995 last Sunday in September at 3 h MESZ

since 1996 last Sunday in October at 3 h MESZ

The Federal Government is not given free reins in the decision on the beginning or the end of the Summer time, but has to follow a European Directive stating that from 2002 onwards Summer Time begins on the last Sunday of March at 1 o’clock UTC in the morning and ends on the last Sunday of October at 1 o’ clock UTC in the morning in every Member state. The data of the daylight savings time are published every five years.

Times of clock settings since 1981:

1981: 29. März 1981, 2 h MEZ bis 27. September 1981, 3 h SZ
1982: 28. März 1982, 2 h MEZ bis 26. September 1982, 3 h SZ
1983: 27. März 1983, 2 h MEZ bis 25. September 1983, 3 h SZ
1984: 25. März 1984, 2 h MEZ bis 30. September 1984, 3 h SZ
1985: 31. März 1985, 2 h MEZ bis 29. September 1985, 3 h SZ
1986: 30. März 1986, 2 h MEZ bis 28. September 1986, 3 h MESZ
1987: 29. März 1987, 2 h MEZ bis 27. September 1987, 3 h MESZ
1988: 27. März 1988, 2 h MEZ bis 25. September 1988, 3 h MESZ
1989: 26. März 1989, 2 h MEZ bis 24. September 1989, 3 h MESZ
1990: 25. März 1990, 2 h MEZ bis 30. September 1990, 3 h MESZ
1991: 31. März 1991, 2 h MEZ bis 29. September 1991, 3 h MESZ
1992: 29. März 1992, 2 h MEZ bis 27. September 1992, 3 h MESZ
1993: 28. März 1993, 2 h MEZ bis 26. September 1993, 3 h MESZ
1994: 27. März 1994, 2 h MEZ bis 25. September 1994, 3 h MESZ
1995: 26. März 1995, 2 h MEZ bis 24. September 1995, 3 h MESZ
1996: 31. März 1996, 2 h MEZ bis 27. Oktober 1996, 3 h MESZ
1997: 30. März 1997, 2 h MEZ bis 26. Oktober 1997, 3 h MESZ
1998: 29. März 1998, 2 h MEZ bis 25. Oktober 1998, 3 h MESZ
1999: 28. März 1999, 2 h MEZ bis 31. Oktober 1999, 3 h MESZ
2000: 26. März 2000, 2 h MEZ bis 29. Oktober 2000, 3 h MESZ
2001: 25. März 2001, 2 h MEZ bis 28. Oktober 2001, 3 h MESZ
2002: 31. März 2002, 2 h MEZ bis 27. Oktober 2002, 3 h MESZ
2003: 30. März 2003, 2 h MEZ bis 26. Oktober 2003, 3 h MESZ
2004: 28. März 2004, 2 h MEZ bis 31. Oktober 2004, 3 h MESZ
2005: 27. März 2005, 2 h MEZ bis 30. Oktober 2005, 3 h MESZ
2006: 26. März 2006, 2 h MEZ bis 29. Oktober 2006, 3 h MESZ
2007: 25. März 2007, 2 h MEZ bis 28. Oktober 2007, 3 h MESZ
2008: 30. März 2008, 2 h MEZ bis 26. Oktober 2008, 3 h MESZ
2009: 29. März 2009, 2 h MEZ bis 25. Oktober 2009, 3 h MESZ
2010: 28. März 2010, 2 h MEZ bis 31. Oktober 2010, 3 h MESZ
2011: 27. März 2011, 2 h MEZ bis 30. Oktober 2011, 3 h MESZ
2012: 25. März 2012, 2 h MEZ bis 28. Oktober 2012, 3 h MESZ
2013: 31. März 2013, 2 h MEZ bis 27. Oktober 2013, 3 h MESZ
2014: 30. März 2014, 2 h MEZ bis 26. Oktober 2014, 3 h MESZ
2015: 29. März 2015, 2 h MEZ bis 25. Oktober 2015, 3 h MESZ
2016: 27. März 2016, 2 h MEZ bis 30. Oktober 2016, 3 h MESZ
2017: 26. März 2017, 2 h MEZ bis 29. Oktober 2017, 3 h MESZ
2018: 25. März 2018, 2 h MEZ bis 28. Oktober 2018, 3 h MESZ
2019: 31. März 2019, 2 h MEZ bis 27. Oktober 2019, 3 h MESZ
2020: 29. März 2020, 2 h MEZ bis 25. Oktober 2020, 3 h MESZ
2021: 28. März 2021, 2 h MEZ bis 31. Oktober 2021, 3 h MESZ