In the PHP documentation, list of supported time zones, UTC
is listed twice:
UTC
Etc/UTC
Is there any conceptual difference between those two, or are they just synonyms?
Firstly, to answer the question:
There is NO difference between UTC
and Etc/UTC
time zones.
Etc/UTC
is a timezone in the Olson-timezone-database (tz database), also known as IANA-timezones-database, in which all timezones conform to a uniform naming convention: Area/Location
.
Since, some timezones cannot be attributed to any Area of the world (i.e. continents or oceans), the special Area Etc
(Etcetera) was introduced. This applies mainly to administrative timezones such as UTC
.
Thus, to conform with the naming convention, the universal coordinated time(zone) is named Etc/UTC
in the tz database.
For administrative timezones other than UTC (e.g. GMT+4
, GMT-8
), the tz database uses POSIX-style signs in the zone-names. POSIX has positive signs for zones that are behind Greenwich (west of Greenwich) and negative signs for zones that are ahead of Greenwich (east of Greenwich).
POSIX-style signs in timezones are the opposite of the definition of timezones in the nowadays widespread and mostly used ISO 8601. In the ISO 8601 timezone format, negative signs indicate a zone is behind UTC (west of Greenwich) and positive signs indicate a zone is ahead of UTC (east of Greenwich). This is what has become the standard usage nowadays.
Possible reasons for the opposite definition in POSIX are: