CONSIDERATIONS ON THE DATE AND TIME

Both the FGDC and ISO standards allow a time to be referred to with a single date or with a time period. For this reason it was decided to implement a scheme based on a time interval with two dates (and times) in which, if the final date is undefined it is considered a single date.

Whenever we refer to a "date" in the broader sense, we are referring to two elements; the date and the hour. It is unusual to need to define the hour in many metadata inputs (like the date on which a database is created) but it is important to define the hour in other cases like, for example, when defining the processes performed on a database (where often a series of processes are performed on the same dataset within a matter of minutes). Another example may be given by images, where the time of acquisition is an important datum. The hour may be defined to the nearest hundreth of a second.

TIME FORMAT

For the case when the hour is defined it is necessary to indicate the reference time system. The solutions are complex and not very satisfactory.

On the one hand it is possible to document all times using the UTC reference (Universal Time Coordinate). This has the advantage that it is an international timescale and it doesn't depend on the time system of the users generating or using the map database. However this system is not very useful for the user as it is uninformative until it is transformed by a series of calculations.

The official local time ("watch time") of the user is the simplest but has some obvious problems at different levels: from duplicated and non-existent times between summer and winter times (in those countries where this system is used) to the misinterpretation of the real time by a user based in another time zone.

For these reasons, and as indicated in the FGDC and ISO standards, where possible both the local time and the differential factor for conversion between this local time and UTC time should be stored.

Knowledge of this factor is not simple since it can vary according to the time of year (in systems where the hour changes for energy-saving summer time the date of this change varies from year to year) and for other reasons (political decisions on the adoption of one or other standard). Frequently indicating the UTC time makes it impossible to return to the local time (although we may know the time system used by the users).

In some cases it may be useful and necessary to store the time in another system: solar time. The solar time depends on the Sun's position in the sky at a particular place. There are two distinct broad approximations for the solar time: the mean solar local time and the apparent/true solar local time, respectively.
* The true solar local time is related to the true position of the Sun, which will vary during the year given by the equation of time (small variations of up to +/- 16 minutes, for example +16m25.9s/-14m14.5s in 2003).
* The solar local time (mean) corresponds to the average position of the Sun (without considering the annual variations given by the equation of time). To obtain the solar local time (mean) it is enough to know the UTC time and the longitude of the region (or its central point) and apply the UTC time correction in hours.

The implemented metadata model allows all three types of dates and times to be considered:

- Official Local Date and Time:

• Without correction: clock time.
• With correction: clock time and an indication of the number of hours and minutes needed to pass from UTC to local time (the time Corrector). For example in Barcelona during winter time: 12:00 offical local time corresponds to 11:00 in UTC time and, therefore, the Corrector is +01:00h. In the same time zone the Corrector may vary depending on the season of the year if the country uses summer and winter times (daylight saving time and standard time). Generally the time corrector is a whole number of hours but in some cases it may be fractions of an hour: for example in Kathmandu (Nepal) the corrector is +05:45 (five hours and forty five minutes).

  When the local time is known together with the corrector it is possible to transform this time to obtain the UTC date but it must be remembered that these changes can lead to a change of day, month or even year. For example, in Alaska (USA) the time corrector is -09:00 (in winter time) so that if the official local time is 19:00h on 28 February 2003, in UTC time will be 04:00h on 1 March 2003.

- UTC Date and Time: Universal Time Coordinates.

- Solar Local Date and Time: mean solar local time.

See the notes on internal format for storing dates and times.