A server is storing the date/time of a client action with System.currentTimeMillis(). Let's say this server is in the EST time zone.
A client in France wants to see what time that action was made. That long value stored via System.currentTimeMillis() (on the US server) is returned to the client in France.
I'm having problems understanding how to make that conversion on the client side to accurately describe their action time. My understanding is that on the server the System.currentTimeMillis() is a zoneless UTC time. Things seem strange when I instantiate a Calendar object with a "UTC" timezone, so is the server storing a time zoned epoch time when it saves with currentTimeMillis()
First attempt:
String timezone = clientTimezone;//Lets say france
Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(time); //This time var is the server stored value (currentTimeMillis)
if(!timezone.equals("")) {
long timezoneAlteredTime = time + TimeZone.getTimeZone(timezone).getRawOffset();
cal = Calendar.getInstance(TimeZone.getTimeZone(timezone));
cal.setTimeInMillis(timezoneAlteredTime);
}
Other attempt:
String timezone = clientTimezone;//Lets say france
TimeZone utc = TimeZone.getTimeZone("UTC");
Calendar cal = Calendar.getInstance(utc);
cal.setTimeInMillis(time); //This time var is the server stored value
if(!timezone.equals("")) {
long timezoneAlteredTime = time + TimeZone.getTimeZone(timezone).getRawOffset();
cal = Calendar.getInstance(TimeZone.getTimeZone(timezone));
cal.setTimeInMillis(timezoneAlteredTime);
}
Am I misinterpreting this issue altogether? How would you go about this conversion
The Answer by Ole V.V. is correct: Call Instant::atZone to adjust a moment from UTC to a particular time zone. Same point on the timeline, different wall-clock time.
Here are a few more thoughts.
Let's say this server is in the EST time zone.
FYI, generally best to set UTC as the time zone on servers.
the System.currentTimeMillis() is a zoneless UTC time
Not zoneless. That method returns the number of milliseconds since the epoch reference of the first moment of 1970 as seen in UTC. (An offset-from-UTC of zero hours-minutes-seconds)
Without the context of an offset-from-UTC or a time zone, you cannot represent a moment, a specific point on the timeline.
Things seem strange when I instantiate a Calendar object
Everything about the Calendar class is strange.
That is why we stopped using that terrible class years ago. Use only the java.time classes.
A server is storing the date/time of a client action with System.currentTimeMillis().
We have a class for that: Instant. So no need to use a mere integer number, now you can use a type-safe and handy class.
The Instant class represents a moment as seen in UTC. Its objects use a resolution of nanoseconds, though most computer clocks nowadays can capture a moment as microseconds.
Instant instant = Instant.now() ;
You can convert between your count-of-milliseconds and Instant.
Instant instant = Instant.ofEpochMilli( yourCountOfMillis ) ;
An Instant is in UTC, by definition. To see the same moment from an on other offset, use OffsetDateTime. To see the same moment in a particular time zone, use ZonedDateTime.
Understand that a time zone is a history of past, present, and future changes to the offset used by the people of a particular region. Politicians frequently change the offset of their jurisdiction(s) for various reasons.
Search Stack Overflow to learn more. These issues have been covered many many times already.
java.time
This is pretty straightforward when you know how. And I recommend that you leave the work to java.time, the modern Java date and time API.
String clientTimezone = "Europe/Paris";
long time = 1_607_708_578_154L;
ZonedDateTime timeInClientTimeZone = Instant.ofEpochMilli(time)
.atZone(ZoneId.of(clientTimezone));
System.out.println(timeInClientTimeZone);
Output from this example code snippet is:
2020-12-11T18:42:58.154+01:00[Europe/Paris]
What went wrong in your code?
My understanding is that on the server the System.currentTimeMillis()
is a zoneless UTC time.
This far your understanding is correct.
Even though the epoch is usually defined in UTC, the clue here is zoneless. So you don’t need any date-time object in UTC. You also don’t need to make any adjustments to the millisecond count, so by doing so, you are turning a correct time into an incorrect one.
Link
Oracle tutorial: Date Time explaining how to use java.time.
Related
Sometimes, we find it is difficult to make judgement, whether to use ZonedDateTime or LocalDateTime, when we want to solve certain date/ time problem.
For instance, given an epoch, we would like to know the day of the week.
We find we can accomplish this task, with either ZonedDateTime or LocalDateTime. Here's the code example
import java.time.*;
public class Main {
public static void main(String[] args) {
long currentTimeMillis = System.currentTimeMillis();
// Yield correct result.
System.out.println("useLocalDateTime -> " + useLocalDateTime(currentTimeMillis));
// Also yield correct result.
System.out.println("useZonedDateTime -> " + useZonedDateTime(currentTimeMillis));
}
public static DayOfWeek useLocalDateTime(long currentTimeMillis) {
LocalDateTime localDateTime = LocalDateTime.ofInstant(
Instant.ofEpochMilli(currentTimeMillis),
ZoneId.systemDefault()
);
DayOfWeek dayOfWeek = localDateTime.getDayOfWeek();
return dayOfWeek;
}
public static DayOfWeek useZonedDateTime(long currentTimeMillis) {
ZonedDateTime zonedDateTime = Instant.ofEpochMilli(currentTimeMillis).atZone(ZoneId.systemDefault());
DayOfWeek dayOfWeek = zonedDateTime.getDayOfWeek();
return dayOfWeek;
}
}
In the above case, is it better to use ZonedDateTime or LocalDateTime? Is there any guideline, so that we can pick up the correct class as tool?
I always have the impression that ZonedDateTime is more "feature rich" than LocalDateTime. Whatever can be accomplished by LocalDateTime, it can be accomplished by ZonedDateTime too, but not vice-versa. Hence, if I get stuck on which to choose, I will go to ZonedDateTime as default. Is that a correct concept?
Do you need to store time data that is attached to a specific time zone, or do you need to process time data that has an associated offset?
If you do, use ZonedDateTime.
If you don't, use LocalDateTime.
Some examples of when I would want to use ZonedDateTime:
I'm parsing an ISO 8601 timestamp with zone information.
I'm looking at data from two different sources located in two physically different locations.
I'm trying to calculate what the day of the week is given a timestamp.
Some examples of when I would want to use LocalDateTime:
I'm assured that my system only needs to care about one time zone - mine.
The data that I'm parsing does not have time stamp information.
I want to know how many seconds have passed between two time stamps. This may get converted to a ZonedDateTime first before it eventualy decants into a Duration if the time stamps are in ISO 8601 format.
Definitely be careful about days of the week across time zones, since the International Date Line can offset the day of the week depending on where you are physically located.
Instead of using System.currentTimeMillis() use ZonedDateTime.now(ZoneId) or Instant.now(). You should almost never need currentTimeMillis() in modern Java. Use the dedicated java.time APIs throughout your application, so that you're working with well-typed data structures instead of primitives like long currentTimeMillis.
given an epoch, we would like to know the day of the week
It's worth recognizing that this isn't a meaningful question without a time zone; at any moment in time there are two (or more?) days of the week in different places on earth. So before we go further we need to ask which time zone(s) do you care about?
Generally speaking, the systemDefault() time zone is not what you want. Instead the caller should provide the time zone they expect. If your program is running locally and only ever needs your machine's clock it may be fine, but the very reason for the split between LocalDateTime and ZonedDateTime is because the system is very often not the correct time zone to be using.
For trivial cases, e.g. a Java process running on your local machine that doesn't care about time zone changes over time, you might correctly use the system time zone. But in such cases it's a good idea to query the system near your main() method and then pass that zone through your application. This makes the application more scalable and testable, if the system zone stops being the right approach down the road.
I have written below code which is running, and giving output. But I'm not sure It's a right one.
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
Date date = new Date();
sdf.setTimeZone(TimeZone.getTimeZone("GMT-7"));
String value = sdf.format(date);
System.out.println(value);
Date date2 = sdf.parse(value);
long result = date2.getTime();
System.out.println(result);
return result;
The above code what I'm trying is, I just need to get the current time of GMT time zone and convert it as epoch format which is gonna used in Oracle db.
Can someone tell me that format, and the above code is right?
First, you should not store time since the epoch as a timestamp in your database. Look into the date-time datatypes that your DMBS offers. In Oracle I think that a date column will be OK. For most other DBMS you would need a datetime column. timestamp and timestamp with timezone may be other and possibly even sounder options depending on your exact requirements.
However, taking your word for it: Getting the number of milliseconds since the epoch is simple when you know how:
long millisecondsSinceEpoch = System.currentTimeMillis();
System.out.println(millisecondsSinceEpoch);
This just printed:
1533458641714
The epoch is defined in UTC, so in this case we need to concern ourselves with no other time zones.
If you needed seconds rather than milliseconds, it’s tempting to divide by 1000. However, doing your own time conversions is a bad habit since the libraries already offers them, and using the appropriate library methods gives clearer, more explanatory and less error-prone code:
long secondsSinceEpoch = Instant.now().getEpochSecond();
System.out.println(secondsSinceEpoch);
1533458641
You said:
I just need to get the current time of GMT time zone…
Again taking your word:
OffsetDateTime currentTimeInUtc = OffsetDateTime.now(ZoneOffset.UTC);
System.out.println(currentTimeInUtc);
long millisecondsSinceEpoch = currentTimeInUtc.toInstant().toEpochMilli();
System.out.println(millisecondsSinceEpoch);
2018-08-05T08:44:01.719265Z
1533458641719
I know that GMT and UTC are not exactly the same, but for most applications they can be (and are) used interchangeably.
Can someone tell me (if) the above code is right?
When I ran your code just now, its output agreed with mine except the milliseconds were rounded down to whole thousands (whole seconds):
1533458641000
Your code has some issues, though:
You are using the old, long out-dated and poorly designed classes SimpleDateFormat, Date and TimeZone. The first in particular has a reputation for being troublesome. Instead we should use java.time, the modern Java date and time API.
Bug: In your format pattern string you are using lowercase hh for hour of day. hh is for hour within AM or PM, from 1 through 12, so will give you incorrect results at least half of the day. Uppercase HH is for hour of day.
Don’t use GMT-7 as a time zone. Use for example America/Los_Angeles. Of course select the time zone that makes sense for your situation. Edit: You said:
I just want to specify the timezone for sanjose. GMT-7 is refer to
sanjose current time.
I believe many places are called San Jose. If you mean San Jose, California, USA, you are going to modify your program to use GMT-8 every time California goes back to standard time and opposite when summer time (DST) begins?? Miserable idea. Use America/Los_Angeles and your program will work all year.
Since you ask for time in the GMT time zone, what are you using GMT-7 for at all?
There is no point that I can see in formatting your Date into a string and parsing it back. Even if you did it correctly, the only result you would get would be to lose your milliseconds since there are no milliseconds in your format (it only has second precision; this also explained the rounding down I observed).
Links
Oracle tutorial: Date Time explaining how to use java.time, the modern Java date and time API.
San Jose, California on Wikipedia
Why not use Calendar class?
public long getEpochTime(){
return Calendar.getInstance(TimeZone.getTimeZone("GMT-7")).getTime().getTime()/1000; //( milliseconds to seconds)
}
It'll return the current Date's Epoch/Unix Timestamp.
Based on Harald's Comment:
public static long getEpochTime(){
return Clock.system(TimeZone.getTimeZone("GMT-7").toZoneId() ).millis()/1000;
}
Here is a solution using the java.time API
ZonedDateTime zdt = LocalDateTime.now().atZone(ZoneId.of("GMT-7"));
long millis = zdt.toInstant().toEpochMilli();
I want to format time like 19:19:00 to different time zones. If I use SimpleDateFormat it always takes into account the start of the epoch: 1970.01.01.
Some timezones have different offsets on the start of the epoch and now. For example, the default offset from Europe/Kiev now is UTC+0200 but in 1970 it was UTC+0300. That means if I run my server under Europe/Kiev the client which login under Europe/Berlin(UTC+0100) will see three hours different instead of two.
I can solve this problem by writing a custom formatter for java.sql.Time. But I want to ask maybe there are some common approach or Java tools/libraries which can solve it.
Another solution can be using joda-time:
TimeZone.setDefault(TimeZone.getTimeZone("Europe/Kiev"));
DateTimeZone.setDefault(DateTimeZone.forID("Europe/Kiev"));
DateTimeFormat.forPattern("HH:mm:ss.SSS")
.withZone(DateTimeZone.forID("Europe/Berlin"))
.print(Time.valueOf("19:00:00").getTime());
You can't format just a time to different time zones. You need a date.
If you want to assume that the date of that time is today, you can try this code:
ZoneId originalZone = ZoneId.of("Europe/Kiev");
ZoneId targetZone = ZoneId.of("Europe/Berlin");
LocalTime originalTime = LocalTime.parse("19:19:00");
LocalTime convertedTime = LocalDate.now(originalZone)
.atTime(originalTime)
.atZone(originalZone)
.withZoneSameInstant(targetZone)
.toLocalTime();
System.out.println(convertedTime);
Is java.time.instant an alternative for you? It handles all Timestamps internally as UTC-Time.
One way to parse it from a string is Instant.parse("2018-05-30T19:00:00")
If you want to have the time for a specific timezone you can get it with myInstant.atZone("Zone")
ZoneId originalZone = ZoneId.of("Europe/Kiev");
ZoneId targetZone = ZoneId.of("Europe/Berlin");
LocalDate assumedDate = LocalDate.now(originalZone);
String formattedTime = assumedDate.atTime(LocalTime.parse("19:19:00"))
.atZone(originalZone)
.withZoneSameInstant(targetZone)
.format(DateTimeFormatter.ofPattern("HH:mm:ss"));
System.out.println(formattedTime);
Today this printed:
18:19:00
When you know the date, you should of course use that instead of just today. In the case of Kyiv and Berlin I think they follow the same rules for summer time (DST), so the precise date may not be important. If converting between zones that don’t use the same transitions, or between a time zone that uses summer time and one that doesn’t, it’s suddenly crucial. And who knows in which of those two countries the politicians will change the rules next year? Better be safe.
Link: Oracle tutorial: Date Time explaining how to use java.time.
I have time 12:00:00 in format HH:mm:ss.
I know that this time comes from server witch is setup with +3 offset.
If i use SimpleDateFormat df = new SimpleDateFormat("HH:mm:ss");, it parses time with regard to device, which can be in a different timezone.
Is there another way to parse it with regard to +3 offset except adding it to the original string?
First, should your server rather send the time in UTC? If clients are everywhere, this would seem more time zone neutral and standardized. However, the way to handle it in code wouldn’t be much different. In any case the server offset form UTC could be constant:
private static final ZoneOffset serverOffset = ZoneOffset.ofHours(3);
In real code you will probably want to make it configurable somehow, though. To parse:
OffsetTime serverTime = LocalTime.parse("12:00:00").atOffset(serverOffset);
System.out.println(serverTime);
This prints
12:00+03:00
Since your time format agrees with LocalTime’s default (ISO 8601), we need no explicit formatter. If a representation of the time with offset is all you need, we’re done. If you need to convert to the user’s local time, to do that reliably you need to decide both a time zone and a date:
LocalTime clientTime = serverTime.atDate(LocalDate.of(2018, Month.JANUARY, 25))
.atZoneSameInstant(ZoneId.of("Indian/Maldives"))
.toLocalTime();
System.out.println(clientTime);
With the chosen day and zone we get
14:00
Please substitute your desired time zone and date.
Just hypothetically, if you knew the user’s offset from UTC, you could use just that:
LocalTime clientTime = serverTime.withOffsetSameInstant(ZoneOffset.of("-08:45"))
.toLocalTime();
The example yields 00:15. However, no one knows when the politicians introduce summer time (DST) or other anomalies in the user’s time zone, so I discourage relying on an offset alone.
And yes, I too am using java.time. SimpleDateFormat is not only long outdated, it is also notoriously troublesome, so java.time is what I warmly recommend.
Set the timezone on your SimpleDateFormat object:
SimpleDateFormat fmt = new SimpleDateFormat("HH:mm:ss");
fmt.setTimeZone(TimeZone.getTimeZone("GMT+03:00"));
I recommend you use the Java 8 date and time API (package java.time) instead of the old API, of which SimpleDateFormat is a part.
Using the Java 8 DateTime API:
DateTimeFormatter formatter = DateTimeFormatter
.ofPattern("HH:mm:ss");
LocalTime clientLocalTime = LocalTime
.parse("12:00:00", formatter)
// Create an OffsetTime object set to the server's +3 offset zone
.atOffset(ZoneOffset.ofHours(3))
// Convert the time from the server timezone to the client's local timezone.
// This expects the time value to be from the same day,
// otherwise the local timezone offset may be incorrect.
.withOffsetSameInstant(ZoneId.systemDefault().getRules().getOffset(Instant.now()))
// Drop the timezone info - not necessary
.toLocalTime();
i'm building an android application which have a chat.
in this chat i each message to have its time sent signature.
my question is as follow:
lets say that the time in my country is X. my friend is abroad and his time is X minus 7 hours.
i'm sending him a message at 16:00 local time.
i want to avoid the situation that he will get at 09:00 a message which it signature will be 16:00 (which is a time in future if you're looking in the eyes of that friend in his country).
is there a way that in my phone the message will be written as 16:00 and in his phone it will be written as 09:00 ? i there a way to convert a time to a local time ?
System.currentTimeMillis() does give you the number of milliseconds since January 1, 1970 00:00:00 UTC. Date object does not save your local timezone.
You can use DateFormats to convert Dates to Strings in any timezone:
DateFormat df = DateFormat.getTimeInstance();
df.setTimeZone(TimeZone.getTimeZone("gmt"));
String gmtTime = df.format(new Date());
linked response
You should keep all time communications using UTC time. Then localize it for display based on the devices current timezone setting.
Use a long to save your time information as milliseconds since "epoch" (which is January 1, 1970, 00:00:00 GMT). It can be retreived with the Date.getTime() method and new Date objects are easily created using the Date(long millis) constructor. The Date objects are then displayed using the local timezone settings on each device.
EDIT:
Epoch is a defined point in time which is expressed differently in different time zones: 1970-01-01 00:00:00 GMT but
1969-12-31 19:00:00 EST. The timestamp is just the number of milliseconds elapsed since that time. So, for example the timestamp 1341169200 corresponds to 2012-07-01 19:00:00 GMT and 2012-07-01 14:00:00 EST.
You will need to save the time zone which your message will be saved in, and transfer it (or send the unix epoch time) and then on the other side make sure you read it in with the Locale time (using the Android documentation for things like http://developer.android.com/reference/java/util/Calendar.html can help).
Take a look at the answer over here:
https://stackoverflow.com/a/6094475/346232
You need to change the time to UTC and then convert on the device to the timezone.
Avoid java.util.Date/.Calendar
The java.util.Date/.Calendar classes bundled with Java (and Android) are notoriously troublesome, flawed in both design and implementation.
Joda-Time
The Joda-Time library is the way to go. This library inspired the java.time package now built into Java 8 (not available on Android).
UTC
As other answers suggested, the best practice (generally) is to keep your business logic and data storage/communication in UTC time zone (which some think of as no time zone or an "anti" time zone). Adjust to a specific time zone only when expected by the user or data-consumer.
Time Zone
The DateTime class in Joda-Time represents a date-time value along with an assigned time zone.
Note that it is best to specify a time zone in all your operations. Otherwise you will be implicitly relying on the JVM’s current default time zone. This is risky because that zone can change – even at runtime at any moment by any code in any thread of any app running within your app’s JVM. And use proper time zone names, never the 3-4 letter codes.
Example Code
Example code in Joda-Time 2.7.
DateTime sent = DateTime.now( DateTimeZone.getDefault() ) ;
DateTime sentUtc = nowMine.withZone( DateTimeZone.UTC ) ; // Generally, use this for your work, including communicating to other threads and apps and such.
When ready to display to the other user, adjust to the expected time zone.
DateTimeZone zone = DateTimeZone.forID( "America/Montreal" ) ; // Or DateTimeZone.getDefault() if you want to rely on their JVM’s current default. To be absolutely sure of expected time zone, you really must ask the user.
DateTime sentMontréal = sentUtc.withZone( zone );
To generate a textual representation of those date-time objects, search the many Questions and Answers on StackOverflow.com on that subject. Search for terms like "joda" and "DateTimeFormatter" and "DateTimeFormat".