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I have a java program that pulls data from the server and it runs 24/7. I have found that it is not changing the date when it updates to pull data. For example if I open it on 10/28/2016 and run 

Date todayDT = new Date();
String todayD = dFormat.format(todayDT);

it will show 10/28/2016. After mid night passes onto the next day and this runs again it will still show 10/28/2016 rather than 10/29/2016. What is the cause of this?

The only current fix I have for this is to close the program and reopen it. Then it will start pulling the correct date.

Ardel
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    are you calling `new Date()` next day as well? – SMA Oct 28 '16 at 16:48
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    What time zone are you in? – MC Emperor Oct 28 '16 at 16:48
  • Where does it pull data from a time server? – Würgspaß Oct 28 '16 at 16:49
  • new Date() runs every time the data is updated which is about every 15 mins so it should call new Date() for the first time at 12:15 AM. I am in central time. Wurgspab I do not understand your question – Ardel Oct 28 '16 at 16:50
  • Time zone /locale issues are the first thing to check. What is the locale/TZ on the server OS, are you doing anything to force working in UTC, and from what source do you track "midnight"? – Gus Oct 28 '16 at 16:51
  • @Gus server time zone is also central. I am not forcing a time zone. The way I understood new Date() worked is that it pulled the date from the system. Thus after Midnight the system date changes to 10/29/2016 thus when this code runs it should pull that date – Ardel Oct 28 '16 at 16:55
  • You learn the reason if you output time next to the date. – talex Oct 28 '16 at 16:58
  • @talex Fri Oct 28 12:03:52 CDT 2016 ? – Ardel Oct 28 '16 at 17:04
  • @Ardel Is it time that you expect to see? – talex Oct 28 '16 at 17:06
  • @talex yes that is the code I expect to see today. But tomorrow I would expect to see Fri Oct 29 12:03:52 CDT 2016 but it seems it would still print Fri Oct 28 12:03:52 CDT 2016 tomorrow. If I was to close my program and rerun it though it would show Fri Oct 29 12:03:52 CDT 2016 no problem. – Ardel Oct 28 '16 at 17:09
  • @Ardel it is look like you use old instance of date, or some multythreading issue (very unlikely) – talex Oct 28 '16 at 17:11
  • Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/126926/discussion-between-ardel-and-talex). – Ardel Oct 28 '16 at 17:13
  • Without more context it's impossible to help. If the date isn't changing then you're not actually running the code each time, eg it's a static variable initialized once. – Dave Newton Oct 28 '16 at 17:15

1 Answers1

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tl;dr

LocalDate.now( ZoneId.of( "America/Montreal" ) )

Time Zone

For any given moment, the date varies around the globe by time zone. For example, at noon in Auckland NZ it is still “yesterday” in Montréal Québec.

While the date may have rolled over in the time zone in your mind, the date may still be “yesterday” in the JVM’s time zone.

Always specify a time zone. If omitted, the JVM’s current default time zone is implicitly applied. That default can be changed at any time during runtime by any code in the same JVM.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

LocalDate

You are using the troublesome old date-time classes, now legacy, supplanted by the java.time classes.

The LocalDate class represents a date-only value without time-of-day and without time zone.

ZoneId z = ZoneId.of( "America/Montreal" );
LocalDate today = LocalDate.now( z );

The date-time objects do not auto-update with the clock. Every time you want to check the date you must call LocalDate.of again to generate a fresh object with the date as it was at that moment of instantiation.

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

  • Java SE 8 and SE 9 and later
    • Built-in.
    • Part of the standard Java API with a bundled implementation.
    • Java 9 adds some minor features and fixes.
  • Java SE 6 and SE 7
    • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
  • Android

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

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Basil Bourque
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