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ISO 8601 advises numeric representation of dates and times on an internationally agreed basis. It represents elements from the largest to the smallest element: year-month-day:

• Calendar date is the most common date representation. It is:

YYYY-MM-DD

where YYYY is the year in the Gregorian calendar, MM is the month of the year between 01 (January) and 12 (December), and DD is the day of the month between 01 and 31.

Example: 2003-04-01 represents the first day of April in 2003.

• Week date is an alternative date representation used in many commercial and industrial applications. It is:

YYYY-Www-D

where YYYY is the Year in the Gregorian calendar, ww is the week of the year between 01 (the first week) and 52 or 53 (the last week), and D is the day in the week between 1 (Monday) and 7 (Sunday).

Example: 2003-W14-2 represents the second day of the fourteenth week of 2003.

• Time of the day is the time representation, using the 24-hour timekeeping system. It is:

hh:mm:ss

where hh is the number of complete hours that have passed since midnight, mm is the number of complete minutes since the start of the hour, and ss is the number of complete seconds since the start of the minute.

Example: 23:59:59 represents the time one second before midnight.

• Date and time represents a specified time of a specified day. When use is made of the calendar date the representation is:

YYYY-MM-DDThh:mm:ss

where the capital letter T is used to separate the date and time components. Thus, for a very precise date and time, look at this:

Example: 2003-04-01T13:01:02 represents one minute and two seconds after one o'clock in the afternoon of 2003-04-01.

The standard has provisions for:

• the omission of components representing smaller units (seconds, minutes), where such precision is not needed,
• the addition of a decimal fraction to the smallest time unit where higher precision is needed.