To complicate matters, some jurisdictions have used the same timezone abbreviation to mean different UTC offsets at different times; for example, in Moscow example above, this is not necessarily the same as local civil time on that date.One should be wary that the POSIX-style time zone feature can lead to silently accepting bogus input, since there is no check on the reasonableness of the zone abbreviations.For example, versions prior to 8.2, which were case-sensitive in some contexts but not others.) Neither timezone names nor abbreviations are hard-wired into the server; they are obtained from configuration files stored under that introduces the time-of-day units.

(See Section 8.5.1 for how this setting also affects interpretation of input values.) Table 8-15 shows an example.

Time zones, and time-zone conventions, are influenced by political decisions, not just earth geometry.

Time zones around the world became somewhat standardized during the 1900s, but continue to be prone to arbitrary changes, particularly with respect to daylight-savings rules.

uses the widely-used IANA (Olson) time zone database for information about historical time zone rules.

For times in the future, the assumption is that the latest known rules for a given time zone will continue to be observed indefinitely far into the future.

In short, this is the difference between abbreviations and full names: abbreviations represent a specific offset from UTC, whereas many of the full names imply a local daylight-savings time rule, and so have two possible UTC offsets.

As an example, specifies noon Eastern Standard Time (UTC-5), regardless of whether daylight savings was nominally in effect on that date.

values are implemented using floating-point numbers, microsecond precision is achieved for dates within a few years of 2000-01-01, but the precision degrades for dates further away.

Note that using floating-point datetimes allows a larger range of standard requires.