I know that I can get browser timezone name: moment.tz.guess()
How can I guess timezone for custom date? e.g. moment("2020-12-30T14:17:40+11:00").tz.guess() ?
You can't. There are 9 different location-based zones that use +11:00 year round, and 5 more that use it for daylight saving time.
You might be able to eliminate a few if the date and time put it into a time zone where daylight time is not in effect and the standard time doesn't match, but from there, the best you could do would be to pick one at random.
See "time zone != offset" in the timezone tag wiki.
Moment-timezone can guess the browser's time zone because it can query the offset for multiple different timestamps, algorithmically reducing the possibilities. Even then, there are often several that could match, and thus the most likely one is picked (based on population statistics). In the end, it's still a guess.
Related
I feel like I'm going round the houses on this one.
In summary, I have a Location model which has opening hours, modelled as opens_at and closes_at in the DB.
All times are stored as UTC in the database.
If I create a location in New York with opens_at set at 08:30 in my form and it gets saved to the DB as 08:30 UTC, why would I need to worry about timezones? I don't want to display the time in local time because the user would then see opening hours of 03:30.
Am I overthinking this? Do I only need to worry about timezones if I have users in multiple countries attending the same event at the same time?
As this is about physical locations, it feels to me that it wouldn't be needed.
If I create a location in New York with opens_at set at 08:30 in my form and it gets saved to the DB as 08:30 UTC, why would I need to worry about timezones?
Because 08:30 in New York is not 08:30 UTC. It's not even the same time for all dates.
It's 13:30 UTC when New York is observing Eastern Standard Time (EST, UTC-5)
It's 12:30 UTC when New York is observing Eastern Daylight Time (EDT, UTC-4)
So, if the location opens at the same time every day, then you must store it in local time. Otherwise you'll be off by an hour one way or the other following the next DST transition.
This can be an issue even for time zones that don't use DST, and even for non-recurring events, because the various governments of the world control the time zones within their borders. They can change whether to start observing DST or stop observing it, the dates and times that DST transitions occur, or their standard offset from UTC. Some give adequate notice when they make such changes, and others do not. There's always some non-zero chance that a given time zone might change its behavior between when you initially recorded the event and when the event takes place. If you recorded using local time, all you need to do is update the time zone data on your systems (which often happens automatically). However, if you recorded the event using UTC then you could be off when converting back after such a change.
In general, future events should always be stored in terms that the event is described - which is almost always in local time of some particular time zone. Store the IANA time zone ID (e.g., "America/New_York") with the location or event so that you can convert when you need to, but converting to UTC prematurely can lead to losing sight of the original information.
Save the "Always UTC" mindset for past or present events, such as timestamping a transaction.
Yes, I actually have run into this. If the user is only going to see the time in the local time zone, then it is not really necessary to save in UTC with time zone. You can save it in local time without time zone.
Reference: Postgres Time without Time Zone
Some developers will say that you should save it in UTC with the time zone, but I can see two arguments against it:
If you don't need it now and you may never need it.
You may not have access to the Time Zone now or may need extra code/complexity to implement and validate.
You can probably add it later if you know the time zone or lat/lon or address.
I'm speccing an application that displays time periods to the user. The goal is to present periods in a simple view (no time, no timezones) and detailed view (date and time, with timezone data). The simple view should be unambiguous, in other words the user can glance at it and their assumptions about what they see are correct (they are valid in the local timezone).
For the end of the global period, displaying the date in the AoE timezone [1] will solve this problem. For example, a submission deadline might display as 2018-04-03 (actually 2018-04-03 23:59:59 AoE). This means submissions are accepted as long as it is April 3 somewhere on the planet.
But I also want to indicate that start of a global period. For example, if submissions open on April 2 2018 00:01, they are accepted as soon as it is April 2 somewhere on the planet. (This would currently be at UTC+14, matching the Line Islands.)
I can't see a way to use AoE to derive a global start time. Is there an equivalent to AoE (a standardized semantic timezone) that tracks the global start time?
Notes:
Hardcoding UTC-12 and UTC+14 is the simple answer for the modern day. But I'm looking for semantic timezones that would be updated if the values changed (and not reference non-existent historical datetimes).
I thought I'd seen Etc/AoE in the tz database but this is not the case.
References:
AoE
UTC-12:00
UTC+14:00
[1] The Anywhere on Earth (AoE) timezone represents the moment a datetime expires "anywhere on Earth". It currently matches time at Howland Island (UTC-12). If a UTC-13 timezone were invented, it would be updated to track that.
As far as I could understand, AoE is not a timezone as defined by IANA (AFAIK, a list of all offsets from some geographic region during history).
It's more like a "concept", an idea of a specific date being valid in any place on earth. As you said, this notion of "being valid" will change if more timezones are created or removed.
I don't even know if date/time API's can properly handle AoE automatically - maybe I should study more. But my conclusion is that the only way to achieve your goal is to check manually:
you could check all available timezones and see if the date is valid there, comparing to the current date/time at that zone
you could configure the UTC+14 as the offset to be compared, and make some scheduled job (daily/weekly/every-time-IANA-publishes-a-new-version?) to check all zones and set the correct one (with the biggest offset?). You must also take care if this zone has Daylight Saving changes, because the offset will change as well (and what to do with overlaps, when clocks shift 1 hour back and a local time may exist twice?)
I have table which have a datetime field named date. When doing a POST in order to insert a new row, the date sent from the client (browser) looks like 2015-11-20T14:30:00+10:00 which is actually a correct date and timezone.
However, inside Postgres this date has been inserted as 2015-11-20 04:30:00.000000, which as you can see, is not at all the same as above. I know the problem is related to the timezone. But I cannot seems to figure out a fix.
For information, I have configured my app timezone :
class Application < Rails::Application
config.time_zone = 'Brisbane'
end
Ideas?
2015-11-20T14:30:00+10:00 means that the local time of 14:30 is 10 hours ahead of UTC. Your database field reflects the correct UTC value of 04:30. This is often the desired behavior, especially if the value represent a timestamp - the date and time something occured (past tense).
In PostgreSQL, there are two different types of timestamp fields (reference)
The TIMESTAMP WITH TIME ZONE field accepts an input that contains a time zone offset. It then converts the value to UTC for storage. On retrieval, it uses the session's timezone setting.
The TIMESTAMP, or TIMESTAMP WITHOUT TIME ZONE simply stores the date and time given, ignoring any offset, and not converting to UTC.
Most of the time, you should indeed use TIMESTAMP WITH TIME ZONE. You should only use TIMESTAMP WITHOUT TIME ZONE if you need to retain the local date and time value, such as in scheduling of future events and calculation of business hours. And for those scenarios, it often makes more sense to split date and time into separate DATE and TIME fields.
One last thing - if you can avoid it, avoid using Rails time zones and use standard tzdb zones. "Australia/Brisbane" is the full tzdb identifier equivalent to the Rails "Brisbane" time zone. Refer to the section on Rails time zones at the bottom of the timezone tag wiki.
I found this gem to be incredibly useful and easy for correctly setting the time https://github.com/kbaum/browser-timezone-rails
I having an column of UNIX time stamp in my database table, which comes from a system that is in the Kuwait time zone.
My database server's time zone is Eastern Time US & Canada. Now I need to convert the UNIX time stamp in to Kuwait time zone date value using an SQL query.
Can anyone tell me how I can convert this UNIX time stamp into a Kuwait time zone date value?
Unix timestamps are integer number of seconds since Jan 1st 1970 UTC.
Assuming you mean you have an integer column in your database with this number, then the time zone of your database server is irrelevant.
First convert the timestamp to a datetime type:
SELECT DATEADD(second, yourTimeStamp, '1970-01-01')
This will be the UTC datetime that corresponds to your timestamp.
Then you need to know how to adjust this value to your target time zone. In much of the world, a single zone can have multiple offsets, due to Daylight Saving Time.
Unfortunately, SQL Server has no ability to work work time zones directly. So if you were, for example, using US Pacific time, you would have no way of knowing if you should subtract 7 hours or 8 hours. Other databases (Oracle, Postgres, MySql, etc.) have built-in ways to handle this, but alas, SQL Server does not. So if you are looking for a general purpose solution, you will need to do one of the following:
Import time zone data into a table, and maintain that table as time zone rules change. Use that table with a bunch of custom logic to resolve the offset for a particular date.
Use xp_regread to get at the Windows registry keys that contain time zone data, and again use a bunch of custom logic to resolve the offset for a particular date. Of course, xp_regread is a bad thing to do, requires certain permissions granted, and is not supported or document.
Write a SQLCLR function that uses the TimeZoneInfo class in .Net. Unfortunately, this requires an "unsafe" SQLCLR assembly, and might cause bad things to happen.
IMHO, none of these approaches are very good, and there is no good solution to doing this directly in SQL. The best solution would be to return the UTC value (either the original integer, or the datetime at UTC) to your calling application code, and do the timezone conversion there instead (with, for example, TimeZoneInfo in .Net or similar mechanisms in other platforms).
HOWEVER - you have lucked out in that Kuwait is (and always has been) in a zone that does not change for Daylight Saving Time. It has always been UTC+03:00. So you can simply add three hours and return the result:
SELECT DATEADD(hour, 3, DATEADD(second, yourTimeStamp, '1970-01-01'))
But do recognize that this is not a general purpose solution that will work in any time zone.
If you wanted, you could return one of the other SQL data types, such as datetimeoffset, but this will only help you reflect that the value is three hours offset to whomever might look at it. It won't make the conversion process any different or better.
Updated Answer
I've created a project for supporting time zones in SQL Server. You can install it from here. Then you can simply convert like so:
SELECT Tzdb.UtcToLocal('2015-07-01 00:00:00', 'Asia/Kuwait')
You can use any time zone from the IANA tz database, including those that use daylight saving time.
You can still use the method I showed above to convert from a unix timestamp. Putting them both together:
SELECT Tzdb.UtcToLocal(DATEADD(second, yourTimeStamp, '1970-01-01'), 'Asia/Kuwait')
Updated Again
With SQL Server 2016, there is now built-in support for time zones with the AT TIME ZONE statement. This is also available in Azure SQL Database (v12).
SELECT DATEADD(second, yourTimeStamp, '1970-01-01') AT TIME ZONE 'Arab Standard Time'
More examples in this announcement.
for a while I´m trying to understand how this timezone times will work, and I had a question:
Today in my country, we are in Daylight saving time (GMT-2).
So the user of my application enter a time, like 11:00AM and post the form.
Far as I know, rails will convert this date to UTC and save in the database (mysql in my case), like: 01:00PM UTC.
When I recover that record, I had to convert to local time to display. Ok?
My question is, lets suppose that this date/time represents a date/time in future, when my country gets out from summer time (GMT-3). Rails will save 01:00PM UTC? Today, in Daylight saving time, how will be the local time? And in the future, how will be this local time?
Basically, I always need to display to user 11:00AM.
thanks.
There are several places where timezone can come into play: the operating system (or probably user account) default setting, the database server, Rails environment.rb.
The key is to make sure all dates are stored with UTC time zone, then displayed in whatever your local timezone is. It sounds like you're doing that.
So your question seems to boil down to "if it's Daylight time, I want to offset by -3 hours, else offset by -2 hours". The Rails time extensions let you determine your current offset like Time.zone.now.utc_offset, and Time#dst? tells you if it's Daylight Savings Time with those two you can conditionally subtract the extra hour (3600 hundred seconds).
7 months after you asked, but perhaps skip_time_zone_conversion_for_attributes= will help - it tells AcitveRecord not to convert timezones on storage or retrieval. See ActiveRecord Timestamp which shows the example:
class Topic < ActiveRecord::Base
self.skip_time_zone_conversion_for_attributes = [:written_on]
end