How can I get the current date and time in GMT from Matlab if it is connected to Internet. I want to make a world clock which calculates time by adding time offset to current GMT.
I believe you can do it without the internet. You can call
time_ms_UTC= java.lang.System.currentTimeMillis;
for the time in milliseconds. You can covert that to a more readable time and use MATLAB's tzoffset to change timezone to GMT.
Disclaimer: I believe this will not give you the international, atomic clock GMT, but local time. I'll leave the answer here for now.
Credit: http://uk.mathworks.com/matlabcentral/newsreader/view_thread/251275
Related
I only want to get the current date object's timezone. The Date class in Swift does not have a way to do so. I have seen ways to convert from one timezone to another. But no way to do so extracting the timezone from a date object.
Why is that?
Is there a way to do so? Thoughts on how? Thanks
Dates don’t have time zones. They’re simply a point in time.
E.g., the moment in which Neil Armstrong stepped foot on the moon. You could describe that moment in UTC, or EST, or any other TZ (or even in other calendar systems), but they’re all referring to the same moment. That would be modelled by a singular date object.
By another analogy, an object has a width, but it doesn't know its unit. Its width can be measured in centimetres, inches, plank lengths or light years. These are all abstractions we put on top of the concept of distance, and they all describe the same thing.
Internally, they’re just a Double that counts the number of secs since the reference date. Clearly, there's no timezone there.
Any time zones you see relating to dates are just an interpretive layer on top of that (e.g. print will call .description, which will format it to your local tz for your convenience.).
RFC 5545 and other standards like JSCalendar define a P1DT12H duration as one nominal day plus 12 exact hours. Normally this will be 36 real-world ("exact" or "accurate") hours, but:
If a Spring DST transition happens during the "one nominal day" part of that duration, then the accurate duration will be only 35 hours.
If a Fall DST transition happens during the "one nominal day" part, then the accurate duration will be 37 hours.
But what if the starting date/time is exactly one nominal day before a discontinuous period? For example, a P1DT12H duration added to 2020-03-07T02:30 in America/Los_Angeles where DST starts at 2020-03-08T02:00. In that case, what should be the calculated local time at the end of that duration?
Is it 2020-03-08T14:30? 2020-03-08T13:30? 2020-03-08T15:30? Something else? Also: why?
The problem is that the naive way of calculating the exact duration would be to add the date portion of the duration using nominal units, then convert that intermediate result to UTC and add the time portion of the duration using exact time. But that intermediate result is an invalid nominal time that's skipped, then the local time of that intermediate value is 2020-03-08T03:30 (3:30AM, not 2:30AM) because RFC 5545 says:
If the local time described does not occur (when changing from standard to daylight time), the DATE-TIME value is interpreted using the UTC offset before the gap in local times.
So using that interpretation of the spec, the final result after adding the 12-exact-hour time portion should be 2020-03-08T15:30 or 3:30PM.
Is this the "correct" answer according to RFC 5455? If not, what should be the answer and why?
Or is this an ambiguity in the standard and there's no objectively correct answer?
I was hoping someone else would answer. Here is my understanding:
Two concepts here:
Either one has the DTEND and is calculating the DURATION, which as you have established, will vary if there is daylight saving change during the event, OR
one has the duration and is calculating the DTEND. It is best to do that in UTC for safety sake.
RE your question:
But what if the starting date/time is exactly one nominal day before a discontinuous period? In that case, what should be the calculated local time at the end of that duration?
For calculating DTEND, nominal day at same time takes us to invalid time. If one uses UTC to calc that nominal day, one gets 3.30 am. The spec says:
In the case of discontinuities in the time scale, such as the change
from standard time to daylight time and back, the computation of the
exact duration requires the subtraction or addition of the change of
duration of the discontinuity.
I understand this to mean yes, when working out the CALCULATED duration (ie where you have DTSTART and DTEND) will vary depending in the events point in the calendar, as you have noted.
RE your question
But that intermediate result is an invalid nominal time that's skipped, then the local time of that intermediate value is 2020-03-08T03:30 (3:30AM, not 2:30AM...."
Yes, however in calculating further I think you went wrong adding the 12H to the local time. Spec says use the earlier UTC offset, which I take to mean use that to get UTC time, use UTC for the calcs, then convert back.
If the local time described does not occur (when changing from
standard to daylight time), the DATE-TIME value is interpreted using
the UTC offset before the gap in local times.
Note this is the UTC offset. So one nominal day takes us to 2.30am which does not 'exist' in LA on 8 March, so we use the UTC offset before the time gap. -8 hours which gives us UTC=10h30.
Plus 12H gives us UTC 22H30.
If we stay with the -8 offset for calculation purposes, we get local time 14:30.
*It is not 100% spelled out in the specification that this is it. More worked examples to confirm would be good.
Advice I have seen elsewhere is to store times in UTC time, do the calcs in UTC time, then for display, calculate local time.*
RE:
Is it 2020-03-08T14:30?
Is this the "correct" answer according to RFC 5455? If not, what should be the answer and why?
I understand it to be 14H30. I cross checked using PHP, with calcs in LosAngeles and in UTC time before DST & during DST, using both datetime->add https://www.php.net/manual/en/datetime.add.php and https://www.php.net/manual/en/datetime.modify.php and consistently got that answer.
I think correct is 2020-03-08T14:30 because if one uses the UTC offset as specified and calcs in UTC, that is what one gets.
PHP Workings
add a nominal day P01D
Before DST:
2020-03-06T02:30:00-08:00
2020-03-07T02:30:00-08:00 with modify
2020-03-07T02:30:00-08:00 add date interval
Over DST:
2020-03-07T02:30:00-08:00
2020-03-08T03:30:00-07:00 with modify
2020-03-08T03:30:00-07:00 add date interval
add a nominal day plus 12 H ie: P01DT12H
Before DST:
2020-03-06T02:30:00-08:00
2020-03-07T14:30:00-08:00 add date interval
Over DST:
2020-03-07T02:30:00-08:00
2020-03-08T14:30:00-07:00 with modify
2020-03-08T14:30:00-07:00 add date interval
For checking offset: https://www.timeanddate.com/worldclock/meetingtime.html?day=8&month=3&year=2020&p1=137&iv=0
Unix time is useful for measuring time, whereas other formats are more useful for telling the time.
This is because (apart from time synchronization), it just ticks forward one second at a time.
It doesn't change when our clock for telling the time has an hour change, for example.
However, there does seem to be one exception. It ignores leap seconds, meaning when there is a leap second, it basically jumps back a second.
I'm wondering is there a similar format to Unix time that also includes leap seconds and has no special cases at all?
Nevermind, unix time has no exception for leap seconds.
I believe the explanation for unix time on Wikipedia is awful:
It is the number of seconds that have elapsed since the Unix epoch, minus leap seconds
This is incorrect, it should be:
It is the number of clock / artificial Earth seconds that have elapsed since the Unix epoch, minus leap seconds
Or in simpler terms:
It is the numbers of measured seconds that have elapsed since the Unix epoch
Hope this solves anyone else's confusion.
Consider a machine whose time is smeared during a leap second with a noon-to-noon linear smear.
I'm wondering how the system clock provides accurate Epoch time during the smear period.
Example:
The leap second is scheduled at 31st dec of 2016.
On the machine, a Unix timestamp at 11:59:00 on 31st of December is 1483185540
At noon the smearing starts, which means the local clock of the system at 1:30 pm is already a few microseconds behind TAI and UTC. The Epoch timestamp should be 1483191000 (exactly 1 hour 31 minutes later), which is not accurate to TAI/UTC anymore since Epoch doesn't respect leap seconds
At 12pm UTC adds an extra second: 11:59:60 pm, the local smeared clock should continue normally
Till, at noon 1st of January global UTC and local UTC sync up again, the local Epoch clock is now an entire second behind global Epoch/TAI
How is this inaccuracy resolved? Does the local Epoch time skip a second once the system knows a leap second happened? Or how is this issue handled?
Does it depend on the implementation of the clock used to calculate the time? If so, how does GNU's coreutils date handle this?
The inaccuracy is not resolved. The Unix Time remains a count of seconds since 1970-01-01 00:00:00 UTC excluding the inserted leap seconds. This has the benefit of making the count of seconds easy to convert to {year, month, day, hour, minute, second} form.
It has the problem that the subtraction of two Unix Time time points that straddle a leap second insertion will result in a time duration that is one second less than reality.
Here's the deal. I have a latitude/longitude set of a location. I need to figure out what the current time is in that location. Here's how I was getting it before:
NSDateFormatter *theFormatter = [[NSDateFormatter alloc] init];
[theFormatter setDateFormat:#"h:mm a"];
NSDate *todaysdate = [NSDate date];
NSString *todaysDate = [theFormatter stringFromDate:todaysdate];
[theFormatter release];
However, I realized that this will give the time for the user's current location. Is there an API somewhere that gives me the time based off of a lat/lon pair?
Thanks in advance.
Unfortunately timezones (and time in general) is never as simple as you would like.
For a simple approximation you can follow jer's suggestion. Longitude ranges from -180 to 180 degrees, there are 24 hours in a day, so you get 15 degrees of longitude per time zone. Center those time zones on 0 degrees longitude so UTC extends from -7.5 to 7.5, UTC+1 is from 7.5 to 22.5, UTC-1 is from -7.5 to -22.5, and so on. You would then have a very simplistic, and wrong, model of how we use time zones.
Take a look at this map of time zones.
Time zones are not well ordered; regions in UTC-1 are adjacent to regions in UTC-3.
UTC-9.5, UTC-4.5, UTC+3.5, UTC+5.75, and UTC+13 are all valid time zones and actively in use.
Once you get that sorted out then you can start to consider daylight savings time.
No, you will have to write one. Since the latitude and longitude points are well known, and timezones are also well known (though change periodically), all you have to do is map where the lat/long boxes are in given timezones, and calculate if your current position is in which one of those boxes. Once you know which box you're in, you'll have enough info to figure out what your time is.
This is not a trivial problem -- but it's not one without a solution. Services like Geonames and WeatherBug are very restrictive and/or costly so don't start developing something before you thoroughly understand their terms of service.
Here are the steps that I followed for my Appcelerator-based iPhone app:
Locate a list of coordinates that describe all timezones as polygons and read them into a SQLite table.
Make a best guess of which timezone the user is in (or is seeking), based on the longitude. This will be several zones.
Pull the polygons of each of the best-guess timezones from the database and run each one through a find-point-in-polygon algorithm. The one that returns true is the correct timezone.
If you a not concerned about Daylight Saving Time, you're done, otherwise, you have a mess to deal with. The landscape of DST is completely illogical and changes frequently. The best I could do was to Google "Daylight saving time rules" (start looking here: Sources for Time Zone and Daylight Saving Time Data) and be prepared to do a lot of work setting up list that you can use for the calculation. Mine still does not work perfectly after a lot of tweaking.
Use this data dump and import it into your program. Then locate the country in which your lat/long exists and it's time zone. You can then calculate what time it is at a specific lat/lon.
Geonames
I've written a small Java class to do this, with the data embedded in the code. It could be easily translated to ObjectiveC. The database is embedded in the code itself. It's accurate to 22km.
https://sites.google.com/a/edval.biz/www/mapping-lat-lng-s-to-timezones