Parsing date with different time zones

Question

Even with about 15 years in Java one always stumbles over the topic of handling dates and times...

Here's the situation: I get a timestamp from some external system as a String representation. The timestamp's semantic is that it represents an UTC date. This timestamp has to be put in an entity and then into a PostgreSQL database in a TIMESTAMP field. Additionally I need to put the same timestamp as local time (in my case CEST) into the entity and then into the database in a TIMESTAMP WITH TIME ZONE field.

What is the right way to ensure that no matter what the settings of the machine executing the code are, the timestamps get stored correctly in the entity (to make some validations with other UTC timestamps) and in the database (to use them in reports later on)?

Here's the code, which worked fine on my local machine:

SimpleDateFormat sdfUTC = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS");
sdfUTC.setTimeZone(TimeZone.getTimeZone("UTC"));
Date utcTimestamp = sdfUTC.parse(utcTimestampString);
// getMachinesTimezone is some internal util method giving the TimeZone object of the machines Location
Calendar localTimestamp = new GregorianCalendar(getMachinesTimezone());
localTimestamp.setTimeInMillis(utcTimestamp.getTime());

But when executing the same code on the server, it resulted in different times, so I assume that it's not the correct way to handle it. Any suggestions?

PS: I read about Joda Time when searching in this forum, but in the given project I'm not able to introduce new libraries since I only change an existing module, so I have to live with the standard JDK1.6

Answer 1

If I understand correctly, You need to set the timezone on the same data/calendar object that you are printing. Like this:

private Locale locale = Locale.US;
private static final String[] tzStrings = {
    "America/New_York",
    "America/Chicago",
    "America/Denver",
    "America/Los_Angeles",
};

  Date now = new Date();
  for ( TimeZone z : zones) {
        DateFormat df = new SimpleDateFormat("K:mm a,z", locale);
        df.setTimeZone(z);
        String result = df.format(now);
        System.out.println(result); 
  }

if i set timezone to SimpleDateFormat it is working fine.

here is the sample code...

String date="05/19/2008 04:30 AM (EST)";
SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy hh:mm aaa (z)");
TimeZone.setDefault(TimeZone.getTimeZone("PST"));
long millis = sdf.parse(date).getTime();
sdf.setTimeZone(TimeZone.getDefault());
System.out.println(sdf.format(new Date(millis)));

Answer 2

I think you have to set the target time zone in you Calendar object. I think something like:

Calendar localTimestamp = new GregorianCalendar(TimeZone.getTimeZone("GMT+10"));
localTimestamp.setTimeInMillis(utcTimestamp.getTime());

In other case Java takes the default system time zone for the Calendar instance.

Answer 3

You can do it by the below example code.

Date date = new Date();

DateFormat formatter = new SimpleDateFormat("dd MMM yyyy HH:mm:ss z");
formatter.setTimeZone(TimeZone.getTimeZone("CET"));

Date date1 = dateformat.parse(formatter.format(date));

// Set the formatter to use a different timezone
formatter.setTimeZone(TimeZone.getTimeZone("IST"));

Date date2 = dateformat.parse(formatter.format(date)); 
// Prints the date in the IST timezone
//    System.out.println(formatter.format(date));