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2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Term Definitions​

UTC (Coordinated Universal Time)​

Coordinated Universal Time as maintained by the Bureau International des Poids et Mesures (BIPM).

second​

A basic unit of time measurement in the International System of Units. It is defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.

minute​

A period of time of 60 seconds. However, see also the Section 5.7 and Appendix D for how leap seconds are denoted within minutes.

hour​

A period of time of 60 minutes.

day​

A period of time of 24 hours.

leap year​

In the Gregorian calendar, a year which has 366 days. A leap year is a year whose number is divisible by 4, but not by 100, unless it is also divisible by 400.

Leap Year Rules:

if (year % 400 == 0) → leap year
else if (year % 100 == 0) → common year
else if (year % 4 == 0) → leap year
else → common year

Examples:

  • 2000: leap year (divisible by 400)
  • 1900: common year (divisible by 100 but not by 400)
  • 2004: leap year (divisible by 4)
  • 2001: common year

ABNF (Augmented Backus-Naur Form)​

A format for representing permissible strings in a protocol or language, as defined in [ABNF].

Email Date/Time Format​

The date/time format used by Internet Mail as defined by RFC 2822 [IMAIL-UPDATE].

Internet Date/Time Format​

The date format defined in section 5 of this document.

Timestamp​

This term is used in this document to refer to an unambiguous representation of some instant in time.

Z​

A suffix which, when applied to a time, denotes a UTC offset of 00:00; often spoken "Zulu" from the ICAO phonetic alphabet representation of the letter "Z".

Example:

2002-07-15T10:30:00Z
Represents: July 15, 2002 at 10:30:00 UTC

Further Information​

For more information regarding time scales, see:

  • Appendix E of [NTP]
  • Section 3 of [ISO8601]
  • The appropriate ITU documents [ITU-R-TF]

Note: Understanding these fundamental terms is critical for correctly implementing and using this specification, particularly the definitions of leap years and leap seconds.

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