Database datetime to java.util.Date conversion with timezone

Question

I have a task of converting, time stored on one of my DB(Sql Server)'s table's column to a specified timezone. The column always contains time in UTC timezone.

The problem I am facing is, when hibernate READS the column and sets it to my entity class, it sets the time in the application server's timezone.

For Eg: if DB has value - 07 Jul 2012 10:30 (which is actually UTC), the hibernate sets the mapped date field as 07 Jul 2012 10:30 PST (assuming the JVM is running at PST).

Now if this date gets converted to any other timezone.. say GMT+5:30, i get unexpected result

To fix the above issue... i wrote the following code

 //Reading the DB time (which does not have timezone info)
 Date dbDate = entityObj.getDBUtcDate();

 //Setting GMT timezone to the date, without modifying the date
 Calendar c = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
 c.set(dbDate.getYear(), dbDate.getMonth(), dbDate.getDate()..., dbDate.getMinutes());

 Date utcDate = c.getTime();

Using above code.. I could get the DB stored date back in UTC timezone, but when I did conversion to some other timezone(say GMT+5:30) using below logic

Calendar outCal = Calendar.getInstance(TimeZone.getTimeZone("GMT+5:30"));
outCal.setTimeInMillis(utcDate.getTime());

Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR, outCal.get(Calendar.YEAR));
cal.set(Calendar.MONTH, outCal.get(Calendar.MONTH));
cal.set(Calendar.DAY_OF_MONTH, outCal.get(Calendar.DAY_OF_MONTH));
cal.set(Calendar.HOUR_OF_DAY, outCal.get(Calendar.HOUR_OF_DAY));                                              
cal.set(Calendar.MINUTE, outCal.get(Calendar.MINUTE));
cal.set(Calendar.SECOND, outCal.get(Calendar.SECOND));
cal.set(Calendar.MILLISECOND, outCal.get(Calendar.MILLISECOND));

//Converted date
Date pstTime = cal.getTime();
//Converted time mill seconds
long timeMilSec = pstTime.getTime();

The time millisecond of the converted date started coming as negative (-54672...), which seems to be representing an invalid time.

My question here is How can i restore the timezone information from DB (without having to have any extra column in DB to specifically store timezone information)?

OR

How can i convert a DB time into a time having a specified timezone(UTC)?

PS: I expect the output in the form of java.util.Date/ Calendar because i need to do one more conversion on this date

Please help me resolving this issue

Answer 1

Dates in Java don't have a time zone. They're just a universal instant in time. If you want to display the date in a given time zone, then simply use a DateFormat initialized with this time zone:

DateFormat df = DateFormat.getInstance();
df.setTimeZone(TimeZone.getTimeZone("UTC"));
System.out.println("The date in the database, in the UTC time zone, is " 
                   + df.format(date));

You don't need to convert anything. The date format prints the appropriate values based on the universal instant it formats, and the time zone you tell it to use.

Similarly, if you want to know if the date is a monday or a tuesday, or if it's 1 o'clock or 2 o'clock, you need to first choose a time zone, convert it to Calendar, and ask the calendar for the information:

Calendar cal = Calendar.getInstance(someTimeZone);
cal.setTime(date);
System.out.println("The day for the date stored in the database, for the time zone "
                   + someTimeZone
                   + " is " + cal.get(Calendar.DATE));

Side note: don't use deprecated methods. They're deprecated for good reasons.