Re: to_timestamp() and timestamp without time zone

On Sunday, June 26, 2011 12:57:15 pm hernan gonzalez wrote:

>

> An instant is a point in the universal time, it's a physical concept,

> unrelated to world calendars. The time point at which the man first landed

> on the moon is an instant, as is the moment at which my server restarted.

> It is not related to a Timezone at all. We can specified it by some

> arbitrary convention (milliseconds passed since the first atomic explosion

> at Hiroshima), or by some human calendar at some place/moment: for

> example, the "wall date and clock used at New York". If (only if) you use

> a Gregorian Calendar to specify/show a instant, you need a date, a time

> and a timezone. (but you have many timezones to choose from - as you have

> several calendars - a timezone is not determined by an instant). A full

> datetime (date, time, timezone) implies an instant - but an instant does

> not imply a timezone.

>

> I suggest to take a look at the Joda time API, which is one of the very few

> date-time API ("key concepts") that is generally though to cover quite

> completely and consistently these issues.

Took you advice and looked up the Joda API definition of an instant:

http://joda-time.sourceforge.net/key_instant.html

"The most frequently used concept in Joda-Time is that of the instant. An Instant is defined as an instant in the datetime continuum specified as a number of milliseconds from 1970-01-01T00:00Z. This definition of milliseconds is consistent with that of the JDK in Date or Calendar. Interoperating between the two APIs is thus simple. "

Look a lot like the Unix Epoch:)

"Within Joda-Time an instant is represented by the ReadableInstant interface. There are four implementations of the interface provided:

• Instant - A simple immutable implementation which is restricted to the UTC time zone and is intended for time zone and calendar neutral data transfer
• DateTime - The most commonly used class in the library, and an immutable representation of a date and time with calendar and time zone
• DateMidnight - Similar to DateTime and also immutable but with the time component forced to be midnight (at the start of a day)
• MutableDateTime - A mutable representation of date and time with calendar and time zone

We recommend the immutable implementations for general usage. "

There are those pesky time zones and calendars again.

--

Adrian Klaver

adrian.klaver@gmail.com