## Question 1:

What years are leap years?

The year 2000, like the years 1996 and 2004, is a leap year - with 29 days in February; but the years 1900, 1999, 2001, 2002, 2003, 2005 and 2100 are not leap years - and have only 28 days in February.

Canada uses the "Gregorian" calendar through the heritage of the British Act of 1750:

"An Act for regulating the Commencement of the Year, and for correcting the Calendar now in use"

(24 Geo. 2 c. 23).

First introduced in 1582 by Pope Gregory XIII as a replacement for the Julian calendar, this calendar is now in worldwide use for civil purposes. The Gregorian calendar's rules for leap years have three parts:

• If divisible evenly by 4, a Gregorian year is a leap year, with a February 29 and 366 days (e.g. 1996/4 = 499, so 1996 is a leap year), UNLESS
• If divisible evenly by 100, a Gregorian year is a normal year with 365 days (e.g.1900/100=19, so 1900 is a normal year of 365 days; as is 2100), UNLESS
• If divisible evenly by 400, a Gregorian year is a leap year; so the year 2000 is a leap year.

These three rules are trying to keep the seasons near fixed dates on the calendar, with the first day of spring (the vernal equinox) near March 21.

Although other rules have been proposed in attempts to improve on the rules of 1582, none has been adopted for civil purposes. The history of the rules is rather involved.

## Question 2:

What is a leap second?

Observations of the sun are the traditional basis for time keeping. History records three times that clocks have become more stable than the timekeeping sun, leading to changes in timekeeping practice. The leap second is the most recent innovation (1972). For the best clocks to stay in alignment with the sun, at irregular intervals it is necessary to insert a leap second - with 6 to 84 months between leap seconds.

With the advent of the first good mechanical clocks, the "sunset to sunset" day was abandoned in favour of the more uniform "noon to noon" day.

As clocks gave rise to chronometers, it became necessary to average-out the predictable variations of the noon-to-noon day and define a reference time scale based on Earth's mean rotation. It has been called variously Universal Time (UT), Zulu time (Z), and Greenwich Mean Time (GMT).

The phrase "mean time" simply implies that an averaging method is being used.

Since the introduction of atomic clocks, which greatly surpass the stability of the rotating Earth, we have technical names for the different averaging methods that address both predictable and random irregularities of Earth's rotation.

#### UT

Universal Time, or UT, is the generic name given to mean solar time on the Greenwich meridian. Often UT is used for civil purposes when it is not necessary to specify the method of averaging. Note that clocks keeping UT (or any of its family members listed below) are not ever adjusted for Daylight Saving Time.

#### UT0

This was the earliest averaging method and simply corrected for the seasonal variation due to the Earth's orbital eccentricity and inclination, using "the equation of time". UT0 is pronounced "U-T-zero" and is the modern way to refer to the first correction method used historically for Greenwich Mean Time.

#### Historical note

Until 1925, two date rollover conventions were used with GMT. For civil purposes the date changed normally, at midnight GMT, but for astronomical purposes it changed at noon (12 hours later) - each convention avoided date changes during its constituency's normal working hours. Historians must be wary about confusing day and night!

#### UT1

Adding the polar wander correction to UT0 gives UT1, the time scale needed for the most accurate celestial navigation and surveying. It was the second method used historically for GMT.

#### UT2

If the seasonal variation of UT1 is averaged out, UT2 results. It was used briefly as a method for GMT and for predicting the rate of UTC before 1972.

#### UTC

If the rate and time are coordinated through international comparisons organized under the Convention of the Metre, UTC results. UTC was used as the final method for GMT by the last time experts at the Royal Greenwich Observatory. UTC, or Coordinated Universal Time is the modern implementation of GMT and is used as the basis for official time around the world.

Until 1972, the duration of the second for each of these time scales varied slightly (but in different ways) to keep in step with variations of Earth's rotation.

#### Leap Seconds

Since 1972 the duration of the second for UTC has been fixed at the value established by an average of atomic clocks around the world (TAI), and leap seconds have been added as required to keep UTC aligned with UT1 within 0.9 seconds.

The International Earth Rotation Service (IERS) in Paris is charged with predicting when the next leap second will be needed. It then informs national time laboratories, such as the National Research Council, of the impending leap second. The leap second can be inserted in (or - if it were ever necessary - removed from) the last second (UTC) of the day, of June 30 or December 31. Clocks which take advantage of the leap second prediction facility, disseminated by the time laboratories, will then have a minute with 61 (or 59) seconds. With a positive leap second, the normal pattern of times changes from

23:58:57, 23:58:58, 23:58:59, 23:59:00, 23:59:01, 23:59:02... to

23:59:57, 23:59:58, 23:59:59, 23:59:60, 00:00:00, 00:00:01...

Other clocks report the same time stamp for two seconds (00:00:00 for example), while others (such as NTP software) gradually slew their clock time to avoid the possibility of reversing the order of two time stamps (for example, 23:59:60.9 and 00:00.1 might be recorded as 00:00:00.9 and 00:00:00.1, and construed from these time stamps as having taken place in the wrong order).

Up-to-date information on leap seconds may be found in our BULLETIN TF-B

## Question 3:

When do the seasons start?

There are four traditional seasons on Earth, marked by the movement of the sun in the sky. For the northern hemisphere:

• Spring starts at the moment when the sun is directly over the equator, going from south to north: the "vernal equinox"'.
• Summer starts at the moment when the sun is farthest north: the "summer solstice".
• Fall starts at the moment when the sun is directly over the equator, going from north to south: the "autumnal equinox".
• Winter starts at the moment when the sun is farthest south: the "winter solstice".

#### The times in this table are Coordinated Universal Time (UTC)

Year Spring Summer Autumn Winter
2000 March 20 07:35 June 21 01:48 Sept 22 17:27 Dec 21 13:37
2001 March 20 13:31 June 21 07:38 Sept 22 23:04 Dec 21 19:21
2002 March 20 19:16 June 21 13:24 Sept 23 04:55 Dec 22 01:14
2003 March 21 01:00 June 21 19:10 Sept 23 10:47 Dec 22 07:04
2004 March 20 06:49 June 21 00:57 Sept 22 16:30 Dec 21 12:42
2005 March 20 12:33 June 21 06:46 Sept 22 22:23 Dec 21 18:35
2006 March 20 18:26 June 21 12:26 Sept 23 04:04 Dec 22 00:22
2007 March 21 00:07 June 21 18:06 Sept 23 09:51 Dec 22 06:08
2008 March 20 05:48 June 20 23:59 Sept 22 15:44 Dec 21 12:04
2009 March 20 11:44 June 21 05:45 Sept 22 21:18 Dec 21 17:47
2010 March 20 17:32 June 21 11:28 Sept 23 03:09 Dec 21 23:38
2011 March 20 23:21 June 21 17:16 Sept 23 09:04 Dec 22 05:30
2012 March 20 05:14 June 20 23:09 Sept 22 14:49 Dec 21 11:11
2013 March 20 11:02 June 21 05:04 Sept 22 20:44 Dec 21 17:11
2014 March 20 16:57 June 21 10:51 Sept 23 02:29 Dec 21 23:02
2015 March 20 22:45 June 21 16:38 Sept 23 08:20 Dec 22 04:48
2016 March 20 04:30 June 20 22:34 Sept 22 14:21 Dec 21 10:44
2017 March 20 10:28 June 21 04:24 Sept 22 22:02 Dec 21 16:28
2018 March 20 16:15 June 21 10:07 Sept 23 01:54 Dec 21 22:22
2019 March 20 21:58 June 21 15:54 Sept 23 07:50 Dec 22 04:14
2020 March 20 03:49 June 20 21:43 Sept 22 13:30 Dec 21 10:02

#### For Canadian time, subtract the following hours from UTC

Pacific Mountain Central Eastern Atlantic Newfoundland 8 7 6 5 4 3.5 7 6 5 4 3 2.5

## Question 4:

What are the sunrise and sunset times for my area this year?

NRC maintains a web page giving sunrise and sunset times for Canadian cities or latitude/longitude positions. Here you will also find other related information.

## Question 5

When does daylight saving time start and end?

Daylight saving time in Canada is determined by provincial legislation. Exceptions may exist in certain municipalities. The time zone maps and the dates listed below have been in effect since 2006. (This has been denoted as serial number #05 as is transmitted in CHU code.)

Province/Territory Start 2nd Sunday in March End 1st Sunday in November
NewfoundlandFootnote 1 02:00 NST 02:00 NDT
Nova Scotia 02:00 AST 02:00 ADT
Prince Edward Island 02:00 AST 02:00 ADT
New Brunswick 02:00 AST 02:00 ADT
QuébecFootnote 2 02:00 EST 02:00 EDT
OntarioFootnote 3 02:00 EST 02:00 EDT
Manitoba 02:00 CST 02:00 CDT
SaskatchewanFootnote 4 02:00 MST 02:00 MDT
Alberta 02:00 MST 02:00 MDT
British Columbia 02:00 PST 02:00 PDT
Northwest Territories 02:00 MST 02:00 MDT
NunavutFootnote 5
Yukon 02:00 PST 02:00 PDT

## Question 6:

How can I get a copy of the Canadian time zone map in postscript (.eps and .pdf) format?

Copies of Canada's time zone maps, are available in postscript format for insertion into your document using a word processor or document editor that supports Adobe postscript. Click on the selection, and respond with Save File (or equivalent) when prompted by Netscape or other browsers. These files are approximately 0.25 to 0.5 Mbytes in size.

## Question 7:

What is the date for Easter Sunday?

In Canada, Easter is a moveable holiday defined in the 1750 legislation:

"An Act for regulating the Commencement of the Year, and for correcting the Calendar now in use"

(24 Geo. 2 c. 23).

The commonly stated rule, that Easter Day is the first Sunday after the Full Moon that occurs next after the vernal equinox, is somewhat misleading because it is not a precise statement of the Act. Easter is determined by the "ecclesiastical moon" defined by tables, which differ somewhat from the real Moon. In addition, the vernal equinox is fixed at March 21, not by the actual position of the sun

The dates of Easter given here are from the Act, the common date used historically by the western Christian churches since the adoption of the Gregorian calendar.

Year Easter Sunday Year Easter Sunday Year Easter Sunday
2000 April 23 2007 April 8 2014 April 20
2001 April 15 2008 March 23 2015 April 5
2002 March 31 2009 April 12 2016 March 27
2003 April 20 2010 April 4 2017 April 16
2004 April 11 2011 April 24 2018 April 1
2005 March 27 2012 April 8 2019 April 21
2006 April 16 2013 March 31 2020 April 12

## Question 8:

Is there a standard for documenting date and time?

International Standard ISO 8601 specifies numeric representations of date and time. The recommended full format is of the form 2001-12-31 23:59:28.73 UTC. The intent of this standard is to avoid confusion in international communications which can arise with the many different national notations. This format has the advantage that it permits dates to be readily sorted in chronological order by computer systems. Further information can be found in the references listed below.

## Question 9:

Why did the 3rd Millennium and the 21st Century start on 1 Jan 2001?

A millennium is an interval of 1000 years and a century is an interval of 100 years. Because there is no year zero, an interval of 1 year has only elapsed since the start of the era, at the end of the year named 1AD. By a similar argument 100 years will only have elapsed at the end of the year 100AD. It is therefore clear that 2000 years had not elapsed until midnight on 31 December 2000. So the 3rd Millennium and the 21st Century began at the same moment, namely zero hours on January 1st 2001.

## Question 10:

What is a Cesium Atomic Clock?

Since the 1950's, NRC has used cesium atomic clocks, which are the world's best timekeepers. They use the exquisite reproducibility of spinning atoms of the element cesium. Pure cesium is a beautiful silver-gold coloured metal that melts just above room temperature. It is uncommon only because it combines so easily with other common elements.

The glass vial in this picture contains a gram of cesium: one year's supply for a typical atomic clock, which does not recirculate the cesium atoms. A gram of cesium could be found in about a cubic foot of ordinary granite. Natural cesium is pure cesium-133 (55 protons and 78 neutrons in the nucleus, 55+78=133): it is non-radioactive.

Cesium-133 atoms are sent from end to end in the vacuum tank of an atomic clock, as illustrated here.

In the clocks at NRC, they travel up to 5 metres at about 250 m/s. NRC's largest cesium clock is shown below.

Drs. Jean-Simon Boulanger and Rob Douglas, Research Officers in the Frequency and Time group, make adjustments to one of the NRC-built cesium atomic clocks. The large aluminum tube is a vacuum vessel which contains the heart of the clock. Cesium atoms are emitted at one end of the tube and pass through two microwave cavities (the copper waveguide which feeds the microwaves to these cavities can be seen above the tube) before they are analyzed and detected at the other end. The clock is located in a copper room to isolate it from radio interference.

#### How does it all work?

Cesium is evaporated at the cesium source to form a beam of well-separated cesium atoms that travel without collisions at about 250 m/s, through a vacuum maintained by the vacuum pump.

The A magnet selects cesium atoms with their atomic magnets pointing one way (those in the f=3 level of the ground state of the cesium-133 atom), and sends other atoms to be absorbed by a carbon getter.

Some atoms have their magnets set spinning by microwaves in the Ramsey cavity. Allowing for tiny corrections, their magnetization spins at 9 192 631 770 rotations per second in a very uniform magnetic field, the C field of less than 1/10 the Earth's magnetic field. Magnetic shielding isolates the atoms from outside magnetic fields. (Quantum mechanics describe these cesium-133 atoms as an oscillating combination of the two hyperfine levels, f=4 and f=3.)

The spinning is stopped by the microwaves at the other end of the Ramsey cavity.

The B magnet collects the cesium atoms that stayed in step with the microwaves, and which now have their magnetization pointing the other way (the cesium-133 atoms in the f=4 level). The B magnet deflects the in-step atoms towards a detector, the hot wire cesium ionizer and ion collector. The other atoms are absorbed by another carbon getter.

The quartz oscillator is adjusted automatically by the servo control to maximize the number of cesium ions collected, keeping the microwaves in step with the spinning of the cesium atoms. After the small remaining biases are measured and eliminated, the output frequency is a very accurate 10000 000 Hz, accurate to about 5 parts in one hundred thousand billion when averaged over a day. This is a frequency standard, suitable for use in metrology, communications, and many other applications in engineering or science.

A cesium atomic clock needs a few other parts. Simple electronics counts the output cycles of the quartz oscillator, and issues a pulse every 10 million cycles - exactly 1 second apart. When first started, the atomic clock's time is set with respect to International Atomic Time (TAI, Temps Atomique International) - which has been kept by generations of atomic clocks since 1958 when it was set relative to astronomical time. Other circuits count the atomic clock's minutes, hours, days, years, decades, centuries, millennia...

## Footnotes

Footnote 1

Prior to November 2011, Newfoundland changed time at 00:01 local time.

Footnote 2

The areas of Québec, east of 63° west longitude, do not change to daylight time and remain on Atlantic Standard time all year round.

Footnote 3

The areas of Ontario, west of 90° west longitude, are in the Central Time zone and change at 2:00 a.m. local time.

Footnote 4

Most of Saskatchewan uses Central Standard time all year round. Areas around Lloydminster are in the Mountain Time zone and change at 2:00 a.m. local time, as in Alberta.