go-pkg/dateparse
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Add better timezone explanation to README.md

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How golang parses date strings with respect to time zones and locations can be really confusing. Document the key points that need to be understood to properly interpret the results of parsing arbitrary date strings, which may or may not have explicit time zone name or offset information include the parsed date string.
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Klondike Dragon 2023-12-14 00:00:36 -07:00
parent 2b3f700718
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@ -9,10 +9,7 @@ Parse many date strings without knowing format in advance. Uses a scanner to re
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**MM/DD/YYYY VS DD/MM/YYYY** Right now this uses mm/dd/yyyy WHEN ambiguous if this is not desired behavior, use `ParseStrict` which will fail on ambiguous date strings.
**Timezones** The location your server is configured affects the results! See example or https://play.golang.org/p/IDHRalIyXh and last paragraph here https://golang.org/pkg/time/#Parse.
**MM/DD/YYYY VS DD/MM/YYYY** Right now this uses mm/dd/yyyy WHEN ambiguous if this is not desired behavior, use `ParseStrict` which will fail on ambiguous date strings. This can be adjusted using the `PreferMonthFirst` parser option.
```go
@ -29,6 +26,20 @@ layout, err := dateparse.ParseFormat("May 8, 2009 5:57:51 PM")
```
Timezone Considerations
----------------------------------
**Timezones** The location your server is configured affects the results! See example or https://play.golang.org/p/IDHRalIyXh and last paragraph here https://golang.org/pkg/time/#Parse.
Important points to understand:
* If you are parsing a date string that does *not* reference a timezone, if you use `Parse` it will assume UTC, or for `ParseIn` it will use the specified location.
* If you are parsing a date string that *does* reference a timezone and *does* specify an explicit offset (e.g., `2012-08-03 13:31:59 -0600 MST`), then it will return a time object with a location that represents a fixed timezone that has the given offset and name (it will not validate that the timezone abbreviation specified in the date string is a potential valid match for the given offset).
* This can lead to some potentially unexpected results, for example consider the date string `2012-08-03 18:31:59.000+00:00 PST` -- this string has an explicit offset of `+00:00` (UTC), and so the returned time will have a location with a zero offset (18:31:59.000 UTC) even though the name of the fixed time zone associated with the returned time is `PST`. Essentially, it will always prioritize an explicit offset as accurate over an explicit
* If you are parsing a date string that *does* reference a timezone but *without* an explicit offset (e.g., `2012-08-03 14:32:59 MST`), then it will only recognize and map the timezone name and add an offset if you are using `ParseIn` and specify a location that knows about the given time zone abbreviation (e.g., in this example, you would need to pass the `America/Denver` location and it will recognize the `MST` and `MDT` time zone names)
* If a time zone abbreviation is recognized based on the passed location, then it will use the appropriate offset, and make any appropriate adjustment for daylight saving time (e.g., in the above example, the parsed time would actually contain a zone name of `MDT` because the date is within the range when daylight savings time is active).
* If a time zone abbreviation is *not* recognized for the passed location, then it will create a fake time zone with a *zero* offset but with the specified name. This requires further processing if you are trying to actually get the correct absolute time in the UTC time zone.
* If you receive a parsed time that has a zero offset but a non-UTC timezone name, then you should use a method to map the (sometimes ambiguous) timezone name (e.g., `"EEG"`) into a location name (e.g., `"Africa/Cairo"` or `"Europe/Bucharest"`), and then reconstruct a new time object with the same date/time/nanosecond but with the properly mapped location. (Do not use the `time.In` method to convert it to the new location, as this will treat the original time as if it was in UTC with a zero offset -- you need to reconstruct the time as if it was constructed with the proper location in the first place.)
cli tool for testing dateformats
----------------------------------