8.5. Date/Time Types
8.5.日期/时间类型
PostgreSQL supports the full set of SQL date and time types, shown in Table 8.9. The operations available on these data types are described in Section 9.9. Dates are counted according to the Gregorian calendar, even in years before that calendar was introduced (see Section B.5 for more information).
Note
注
The SQL standard requires that writing just timestamp be equivalent to timestamp without time zone, and PostgreSQL honors that behavior. timestamptz is accepted as an abbreviation for timestamp with time zone; this is a PostgreSQL extension.
SQL标准要求timestamp表示timestamp without time zone，PostgreSQL严格的遵守了该要求。作为扩展，PostgreSQL中使用timestampz作为timestamp with time zone的简写。
time, timestamp, and interval accept an optional precision value p which specifies the number of fractional digits retained in the seconds field. By default, there is no explicit bound on precision.The allowed range of p is from 0 to 6.
time,timestamp和interval接受可选的精度值p，此值指定在秒字段中保留的小数位数。默认，没有明确的限制。p的允许范围为0到6。
The interval type has an additional option, which is to restrict the set of stored fields by writing one of these phrases:
interval类型还有一个附加选项，它可以通过编写以下短语之一来限制存储字段的集合：
YEAR
MONTH
DAY
HOUR
MINUTE
SECOND
YEAR TO MONTH
DAY TO HOUR
DAY TO MINUTE
DAY TO SECOND
HOUR TO MINUTE
HOUR TO SECOND
MINUTE TO SECOND
Note that if both fields and p are specified, the fields must include SECOND, since the precision applies only to the seconds.
注意，如果同时限定了fields和p，那么fields中必须包含second，因为p仅适用于秒。
The type time with time zone is defined by the SQL standard, but the definition exhibits properties which lead to questionable usefulness. In most cases, a combination of date, time, timestamp without time zone, and timestamp with time zone should provide a complete range of date/time functionality required by any application.
time with time zone时间类型是由SQL标准定义的，但是该定义显示的属性其实用性存在问题。在大多数情况下，date，time，timestamp without time zone和timestamp with time zone的组合使用即可满足应用程序对于日期时间功能的需求。
The types abstime and reltime are lower precision types which are used internally. You are discouraged from using these types in applications; these internal types might disappear in a future release.
abstime和reltime是内部使用的低精度数据类型。不要在应用程序中使用此理性，因为在未来的版本中，可能会丢弃这两种类型。