Date and time standards

In almost all aspects of our lives, it is important that we all agree upon the correct time. This issue is solved by using standards for regulating time. Moreover, we need some way to correctly communicate the date and time information. For example, the date "01-02-03" may have different interpretations based on where we live. Thus, we have to standardize the way we reference dates and time so that there is no misunderstanding.

UTC and UTC-SLS standards

Coordinated Universal Time (UTC) is the world's principal time standard for regulating clocks and time. It has effectively replaced the Greenwich Mean Time (GMT), which is the yearly average of the time at the Royal Observatory Greenwich.

UTC uses the International Atomic Time (IAT) in order to keep the correct time, which is the mean time of 400 atomic clocks around the globe.

Although we can measure time with great accuracy, the Earth's rotating speed isn't that stable. Due to earthquakes, tides, etc., the time we measure isn't synchronized with the astronomical time – that is, what the time should be according to the Earth's position relative to the Sun. This time is corrected by the insertion of leap seconds on an irregular basis. The Coordinated Universal Time with Smoothed Leap Seconds (UTC-SLS) standard is effectively the same as the UTC standard, the difference being that it takes into account the leap seconds correction.

Time zones

UTC provides the correct time for the longitude equal to zero degrees, but the rest of the world has its own local time. Thus, the countries are divided into different Time Zones, whose local time ranges from UTC minus 12 hours to UTC plus 14 hours. The local time of a time zone is represented as UTC+<hours:minutes> or UTC-<hours:minutes>. For example:

UTC-05:00
UTC+00:00
UTC+02:00
UTC+05:45

Time zones may also be denoted by alphabetic abbreviations, for example "MUT" stands for "Mauritius Time", that is UTC+04:00.

Following are some examples of time zones. Note that normally, countries have a single time zone, but some large countries may be divided into several time zones.

In many countries, the local time difference with UTC doesn't remain constant all year round due to the Daylight Saving Time (DST) practice, that is advancing the clocks in spring and then returning them back to normal time in autumn. The adjusted time is called summer time. DST is used for extending the sunlight period into the evening hours and thus saving energy.

The ISO 8601 standard

Various countries have different text notations for date and time, so there may be confusion when date or time data is communicated by people, documents, or information technology systems.

ISO is the abbreviation for the International Organization for Standardization, an international non-governmental organization for issuing various standards. ISO 8601 is an international standard for the text representation of date and time. It is used for the correct date and time data communication and is country-independent.

This standard defines the notation for dates, times, time zones, and time intervals. There are 2 categories of notations: the basic notation, where the minimum required number of characters is used, and the extended notation, which is more human-friendly.

The ISO 8601 date and time representation

A date is formatted as YYYY-MM-DD (extended) or YYYYMMDD (basic), where Y denotes the year digits, M denotes the month digits, and D denotes the day digits. For example:

2000-01-01
2001-12-31
20100207
20121120

Time is formatted as Thh:mm:ss.fff, Thh:mm:ss (extended), Thhmmss or Thhmmss.fff (basic), where h denotes the hour digits, m the minutes digits, s the seconds, and f the seconds' decimal digits (depending on the accuracy). T is used as an indicator that the string that follows represents time. Also, note that midnight may only be referred as 00:00:00 and not 24:00:00. For example:

T12:00:15
T120015
T14:01:34.55
14:01:34.55
T170517.3432
T00:00:00
00:00:00

The time zone indicator is formatted as Z (for UTC standard time), ±hh:mm, ±hhmm, or ±hh. For example:

Z
+07:00
+0700
-02:00
-02

All time data have a fixed number of digits, and if the actual numbers have less digits, then they have to be padded with leading zeros. For example, the correct date format is 2022-02-02 and not 2022-2-2.

Date and time together are represented in the format of <date>T<time><time zone indicator>. The time indicator T cannot be omitted here, while the time zone indicator is optional.

Following are some examples of the ISO 8601 date-time representation:

2022-02-16T16:31:03+02:00
2022-02-16T16:31:03Z
20220216T163103Z

The ISO 8601 duration representation

Time duration is represented as P<n>Y<n>M<n>DT<n>H<n>M<n>S, where P denotes that this string represents time duration, Y denotes years, M months, D days, T is the days-time divider, H denotes hours, the M after T denotes minutes, and S – seconds. <n> denotes the value of the time element that follows it. For example:

P2Y11M5DT4H10M3S

The string denotes the duration of 2 years, 11 months, 5 days, 4 hours, 10 minutes, and 3 seconds.

If the value of a time item is zero, it can be omitted. For example:

P4Y      Duration of 4 years
PT3H5M   Duration of 3 hours and 5 minutes
P7DT6H   Duration of 7 days and 6 hours

The last item may also have decimal digits. For example:

P12.5Y         Duration of 12.5 years
PT2H3M10.23S   Duration of 2 hours, 3 minutes and 10.23 seconds
P2M7.3D        Duration of 2 months and 7.3 days

The ISO 8601 time interval representation

Time intervals can be expressed in the following formats:

1) <Starting date-time>/<Ending date-time>

For example:

2022-12-01T00:00:00Z/2022-12-31T23:59:59Z
2000-01-01T00:00:00+03:00/2011-01-01T00:00:00+03:00
2022-02-01T14:00:00-05:00/2022-02-01T14:15:59-05:00

2) <Starting date-time>/<Duration>

For example:

2000-01-01T00:00:00Z/P3Y10M4DT2H11M13S
2012-07-19T17:12:00Z/PT35M

3) <Duration>/<Ending date-time>

For example:

P1Y1M2DT21H9M7S/2000-01-01T00:00:00Z
P3Y/2021-01-01T01:00:00Z

Conclusion

In this topic, we've introduced 2 important international standards concerning time: UTC and ISO 8601. The former is used for keeping and regulating time, while the latter standardizes the representation of date and time formats. Although they target completely different aspects of time, together they make it possible for all of us and all information systems to communicate the correct time.