how can we convert the GMT to local SQL DATE?
When i store GMT time , then 1 hour is subtracted from this value.
please help me.
Your question is a bit ambiguous as written, but I am guessing that you want to convert a UTC datetime (sometimes called GMT or Zulu time) into a datetime in your local timezone.
You indicate that your local time zone is -1 hour offset from UTC time.
In this case you simply use the DATEADD function:
SET MyTimeLocal = DATEADD(HH, -1, MyTimeGMT)
Note that this will only work so long as your time zone really is one hour before UTC time. If your location uses daylight saving time, this will be wrong as soon as the time changes, and it will be extra wrong when the time to be converted falls in the crack of the clock change.
There are many better ways to do this in general, but to give a general solution we would need to know what version of SQL is being used and what exactly you are trying to accomplish.
Related
I trying to parse a date in ISO 8601 format and some moments aren't clear for me.
For example, I have the next date: 2020-04-16T07:16:34.858215+03:00 in Europe/Moscow timezone.
Does it mean 07:16 in Moscow time or 10:16? I mean do I need to add 3 hours to date or date is in Moscow time already and timezone just shows how it diffs from UTC?
P.S. I tried to find information about it but everywhere is just common format description without details.
The time specified is the local time, so 2020-04-16T07:16:34.858215+03:00 means 7:16am in Moscow, or 4:16am UTC.
Wikipedia has a good example which clarifies things to at least some extent:
The following times all refer to the same moment: "18:30Z", "22:30+04", "1130−0700", and "15:00−03:30". Nautical time zone letters are not used with the exception of Z. To calculate UTC time one has to subtract the offset from the local time, e.g. for "15:00−03:30" do 15:00 − (−03:30) to get 18:30 UTC.
It's really unfortunate that ISO-8601 talks about this as a time zone, when it's only a UTC offset - it definitely doesn't specify the actual time zone. (So you can't tell what the local time will be one minute later, for example.)
Background
I am investigating different use cases of moment.js for a project but am stumped on the issue of daylight savings ending in the fall. Before asking my question, since I want to be clear and provide background for others who have similar questions, let me explain what I am doing and what found with spring daylight savings.
First, I am working with UTC timestamps and America/New_York timestamps. In the US, daylight savings in 2017 begins on March 12 at 2AM (skipping from 2:00:00AM to 3:00:00AM) and ends on November 5 at 2AM (reverting from 2:00:00AM to 1:00:00AM). Since I also always know the target time zone (America/New_York) that I need to convert to, I will not rely upon moment.js to detect my local time zone and instead explicitly specify the time zone I want.
During the spring when daylight savings goes into effect, the time zones that observe daylight savings, like America/New_York, jump forward an hour. moment.js handles this just fine.
For example, if I pass moment.js a UTC timestamp for the time one second before daylight savings goes into effect for America/New_York, it looks like this:
moment('2017-03-12T06:59:59Z').tz('America/New_York').format('YYYY/MM/DD hh:mm:ss a z')
The input above is taken as UTC because of the Z on my timestamp and I am explicitly setting the target timezone with .tz('America/New_York') so that it doesn't use local system time.
Alternatively using moment-timezone I can explicitly set the input time zone as UTC and set the output as America/New_York.
moment.tz('2017-03-12T06:59:59', 'UTC').tz('America/New_York').format('YYYY/MM/DD hh:mm:ss a z')
Either way, the result is 2017/03/12 01:59:59 am EST.
Then, I run the same commands for a moment just one second later. I will just use the format given in the first example above where I specify the time as UTC and then convert it to America/New_York time:
moment('2017-03-12T07:00:00Z').tz('America/New_York').format('YYYY/MM/DD hh:mm:ss a z')
And my result is correct as expected: 2017/03/12 03:00:00 am EDT - due to daylight savings the time skipped ahead by one hour.
I can then use moment-timezone to go back the other way by passing in an America/New_York timestamp and converting it to UTC.
moment.tz('2017-03-12T01:59:59', 'America/New_York').utc().format('YYYY/MM/DD hh:mm:ss a z')
This gives me 2017/03/12 06:59:59 am UTC
And the next moment in the America/New_York time zone, because of daylight savings coming into play, is 03:00:00 so I convert that to UTC...
moment.tz('2017-03-12T03:00:00', 'America/New_York').utc().format('YYYY/MM/DD hh:mm:ss a z')
... and get 2017/03/12 07:00:00 am UTC which looks correct.
An hour was skipped ("lost") in America/New_York time, but moment can detect that and convert it to UTC.
In Summary, for the spring daylight savings change in the US, I can pass a UTC timestamp into either moment.js or moment-timezone and get back a timestamp in another time zone with the correct daylight savings offset also applied. I can also then pass an America/New_York timestamp into moment and get back a correctly converted UTC timestamp.
My Question
Great, so I want to do the same thing when daylight savings ends in the fall and of course it isn't that simple. My hypothesis is that due to daylight savings effectively causing an hour to "repeat", there is no way for moment to know the correct time UTC. In other words, where there was a gap of one hour when daylight savings started, now we have overlap of one hour.
Question (part 1): Is there a way to pass a relative timestamp and time zone into moment and get back the correct UTC time? When I tried this in the examples below, moment skips an hour on the UTC side.
moment.tz('2017-11-05T01:00:00', 'America/Denver').tz('UTC').format('YYYY/MM/DD hh:mm:ss a z') => "2017/11/05 07:00:00 am UTC"
moment.tz('2017-11-05T01:59:59', 'America/Denver').tz('UTC').format('YYYY/MM/DD hh:mm:ss a z') => "2017/11/05 07:59:59 am UTC"
moment.tz('2017-11-05T02:00:00', 'America/Denver').tz('UTC').format('YYYY/MM/DD hh:mm:ss a z') => "2017/11/05 09:00:00 am UTC"
moment.tz('2017-11-05T02:59:59', 'America/Denver').tz('UTC').format('YYYY/MM/DD hh:mm:ss a z') => "2017/11/05 09:59:59 am UTC"
I assume its because time is happening chronologically like in the example below. UTC offset context is not given therefore I assume that moment cant distinguish between the America/New_York timestamps preceded by asterisks:
*2017/11/05 01:00:00 am America/New_York => 2017/11/05 07:00:00 am UTC
*2017/11/05 01:59:59 am America/New_York => 2017/11/05 07:59:59 am UTC
*2017/11/05 01:00:00 am America/New_York => ???
*2017/11/05 01:59:59 am America/New_York => ???
2017/11/05 02:00:00 am America/New_York => 2017/11/05 09:00:00 am UTC
2017/11/05 02:59:59 am America/New_York => 2017/11/05 09:59:59 am UTC
Again, what I want to know is if there is a way around this? Currently the timestamps in the data that I have does not contain UTC offsets.
Question (part 2): If I am presenting data in the America/New_York time zone then is it correct to think that I will essentially have two hours of data points all stuffed into a (seemingly) single one hour period from 01:00:00 to 01:59:59 on November 5, 2017?
Related Topics
There are a few other topics on SO that are related to this but none that I have found pose or answer this same question. I will link a few here for reference:
Moment.js Convert Local time to UTC time does work
Initialize a Moment with the timezone offset that I created it with
It looks like you have thought the problem through and done some basic research. Thanks!
What you are describing is covered in the moment-timezone docs here. If the data isn't available as a UTC offset in your input, there's no way to tell the difference between the first or second occurrence of an ambiguous local time. Moment picks the first occurrence, because time moves in a forward direction, so this is usually the most sensible choice for most scenarios.
The problem is one of ambiguity. Even as just a human being, if I say "1:00 am on November 5th 2017 in New York" you don't know which of two points in time I'm describing.
That said, sometimes you have external knowledge that can help. For example, if you have an ordered set of timestamps containing time that skips backwards, then you know you encountered a fall-back transition. Say I'm recording data at 15 minute intervals in local time:
00:45
01:00
01:15
01:30
01:45
01:00 <--- this one comes next sequentially, but appears backwards, so infer transition
01:15
01:30
01:45
02:00
You'll have to write your own detection logic to compare one value to the next for that scenario. Also note that if you don't have any time that appears to be out of sequence, then you cannot be assured of which occurrence is being described. A "heartbeat" signal can assist with this in some scenarios.
Now how do you choose the second occurrence in Moment without knowing the offsets in advance? Like this:
First, grab the hasAmbiguousWallTime function from here.
Then define another function:
function adjustToLaterWhenAmbiguous(m) {
if (hasAmbiguousWallTime(m)) {
m.utcOffset(moment(m).add(1, 'hour').utcOffset(), true);
}
}
Now you can do this:
// start with the first occurrence
var m = moment.tz("2017-11-05T01:00:00", "America/New_York");
m.format(); // "2017-11-05T01:00:00-04:00"
// now shift it to the second occurrence
if (... your logic, such as wall time going backwards in sequence, etc. ...) {
adjustToLaterWhenAmbiguous(m);
m.format(); // "2017-11-05T01:00:00-05:00"
}
These two functions should probably be hardened and added to moment-timezone, but they should be sufficient for the scenario you describe.
A couple of other minor points:
Instead of moment.tz(s, 'UTC'), consider using moment.utc(s)
Instead of moment.tz(s, 'America/New_York').tz('UTC'), consider
using moment.tz(s, 'America/New_York').utc()
You may want to review the DST tag wiki for visualization of the problem space.
On part two of your question, yes - you'll end up stuffing two hours of data into what could possibly be visualized as a one-hour space. People have this problem with graphs and charts all the time. They graph something with a constant value over local time, then see a zeroing effect in the spring, and a doubling effect in the fall. Even if you tell moment to use the later occurrence, you won't avoid this unless you actually display the graph in UTC instead of local time.
Some countries are observing daylight saving, but I want to get the timezone name irrespective of daylight saving.
In short I want standard time of such countries instead of daylight saving time.
Is there any way to convert this or anything else?
P.S. I want this because of my server requirement.
//----------------------------------------------------------------
I have seen this reference in apple.
+ (instancetype)timeZoneForSecondsFromGMT:(NSInteger)seconds
Discussion The name of the new time zone is GMT +/– the offset, in
hours and minutes. Time zones created with this method never have
daylight savings, and the offset is constant no matter the date.
But I wonder how it will useful to me for my problem? As I want timezone name without daylight saving not any time with offset.
If you just need the time zone with respect to GMT without accounting for DST, the simplest option would be to:
Find the DST offset using 'daylightSavingTimeOffset' on your suspect time zone.
Subtract that value from the timezone's GMT offset.
Create a new time zone with the now adjusted offset.
If you need the name of the standard time zone, the simplest approach might just be to look for "DST" and "Daylight" in the time zone description and then pick the time zone that has the greatest number of words matching to the DST time zone. That might not be the most elegant solution, but it would work for most time zones.
It was actually this
typedef enum : NSInteger {
NSTimeZoneNameStyleStandard,
NSTimeZoneNameStyleShortStandard,
NSTimeZoneNameStyleDaylightSaving,
NSTimeZoneNameStyleShortDaylightSaving,
NSTimeZoneNameStyleGeneric,
NSTimeZoneNameStyleShortGeneric
} NSTimeZoneNameStyle;
The enum answered it for me. My bad , I should have read this earlier.
Not sure if the title of my question was accurate so sorry if it's misleading, here goes.
I am doing some work with that involves timezones and i just want to make sure i get this right... if i want something to start at 03:00:00 my time and my timezone offset is -5 all i need to do is add 5 to 03:00:00 giving me 08:00:00 and that is the UTC time?
It depends what you mean by "timezone offset". Usually an offset is expressed as the amount added to UTC to get to local time, in which case you need to subtract it from the local time in order to get back to UTC (so it would be 22:00 on the previous day in your case).
So for example, Pacific Daylight Time has an offset of -7 - it's 7 hours behind UTC.
However, there are situations (annoyingly) where the offset is expressed the other way round, so make sure you know which way is appropriate for your specific context.
Note that knowing the offset doesn't mean you know the time zone - there can be multiple time zones with the same offset for a particular moment, but different rules for when the offset changes.
...Yes. Depending on what language you're doing it in, it may or may not be as easy as writing
time-offset;
I finally found out the difference between UTC and GMT by making the effort to look it up on Wikipedia today. Technically speaking it appears that GMT != UTC because you do not know if it is UTC or UT1 being referred to. However practically, people use the terms interchangeably to indicate the same timezone.
A while ago, I suggested that we change the user interface of one of my companies apps to display UTC instead of GMT.
Just to be sure that our database was not calculating the potential seconds difference between GMT and UTC, I ran the below query and verified that they both are just acting as aliases for the same timezone.
select now() AT TIME ZONE 'GMT', now() AT TIME ZONE 'UTC';
timezone | timezone
----------------------------+----------------------------
2009-02-11 08:46:11.643032 | 2009-02-11 08:46:11.643032
(1 row)
What do you think? Do enough users out there understand UTC? Is it better to use the older but more common term? Or should I just do a UTC/GMT?
Normal humans don't need to worry about the few seconds difference between GMT and UTC. The difference only matters to astronomers and time nerds.
I have seen very little software that bothers to make the distinction. Most software ends up using the labels "GMT" and "UTC" interchangeably. Typically it just means "clock time after removing the local time zone offset in exact hours (or half/quarter hours)."
In most cases, nobody will be concerned about the sub-second technical difference between GMT and UTC.
However, writing that the time is expressed in UTC instead of GMT avoids one source of confusion:
Greenwich (and the UK in general) is currently GMT+01:00 because of the daylight saving time (DST).
GMT+01:00 does not mean 1 hour ahead of the time in the UK as one could mistakenly think. Because of the DST, GMT+01:00 is currently the exact time in England.
Stating it as UTC+01:00 helps to avoid this confusion.
Personally, I think of the term UTC before I think of GMT.
I think of GMT before UTC, but I am also living at GMT (+/-0)