Local Time and Universal Time

Local time is the time in your time zone or longitude. Two people in two different locations may not necessarily have the same local time, although they may have, as, for example New Caledonia and Vanuatu, which are approximately 500 km apart, but have the same local time.

As seismic events generally affect areas spanning several time zones, a standard time is used to describe them to avoid ambiguity, ie universal time co-ordinated (UTC). There are many other names for it, such as Universal Time or UT and GMT or Greenwich Mean Time, ie at the Greenwich Meridian. The term Greenwich Time is falling out of use.

The Prime Meridian has been confused with the reference time zone, but that is purely a matter of convention and they can be dissociated.

Unlike latitude, in which zero point, ie the equator, does not raise any issues, zero longitude has to be defined by convention and it is now the Greenwich Meridian that is used for all lines of longitude. This has not always been the case, as in the 18th century, the British used the Greenwich Meridian and the French the Paris Meridian.

Thus, when talking about the moment when the seismic event occurred, UTC time is always given first and the local date can sometimes be given afterwards. This approach also does away with the difficulty of daylight saving time as used in some countries, such as Australia and New Zealand, but not in others, such as New Caledonia or Vanuatu.

In our form, when we ask you the date, please indicate the local date so as to prevent confusion. We will convert it ourselves.