I have read all of the other Q/A about Date Manipulation
No, you certainly did not read them all.
- You would have learned that both the legacy date-time classes (such as
java.util.Date & java.util.Calendar) and the Joda-Time project are supplanted by the java.time classes (1,890 results for search on 'java.time').
- You would have learned not to track date-time values as a count-from-epoch. Debugging and logging becomes very difficult with bugs going undiscovered as humans cannot decipher the meaning of a long integer as a date-time. And because many granularities of counting (whole seconds, milliseconds, microseconds, nanoseconds, whole days, and more) and at least a couple dozen of epochs are employed in various software projects create ambiguity about your data with assumptions leading to errors, misinterpretation, and confusion.
- You would have learned to use date-time types in your database to track date-time values.
- You would have learned to work and store date-time values in UTC. Adjust into a time zone only where required by logic or as expected by the user for presentation. “Think global, present local.”
- You would have learned that while a valiant industry-first effort, the legacy date-time classes are poorly designed, confusing, and troublesome. See What's wrong with Java Date & Time API? for some discussion. Joda-Time was the first good date-time library in the industry, and inspired its replacement, the java.time classes built into Java 8 and later.
I'll be somewhat brief as all of this has been covered many times already on Stack Overflow.
Work in UTC. In Java that means the Instant class is commonly used. The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).
Instant instant = Instant.now();
Any serious database such as Postgres tracks date-time values in UTC. Your JDBC driver handles the detail of converting from database internally-stored data to a Java type. JDBC drivers that comply with JDBC 4.2 and later can directly handle java.time types via PreparedStatement::setObject & ResultSet::getObject methods.
myPreparedStatement.setObject( … , instant );
For non-compliant drivers, fall back to using java.sql types such as java.sql.Timestamp to communicate with database, and convert to/from java.time types via new methods added to the old classes. The internal details of how the database handles date-time values may be quite different than how java.time does. For the most part the JDBC driver hides all the nitty-gritty details from you. But one critical issue is resolution, which you should study in your database. The java.time classes handle date-times with a resolution up to nanoseconds but your database may not. For example, Postgres uses a resolution of microseconds. So going back-and-forth means data-loss. You want to use the truncation methods on the java.time classes to match your database.
myPreparedStatement.setTimestamp( … , java.sql.Timestamp.from( instant ) );
So, no time zone involved. So no “worrying about time-zones while handling database Date”.
When you want to see the same moment through the lens of a region’s wall-clock time, apply a ZoneId to get a ZonedDateTime.
ZoneId z = ZoneId.of( "Asia/Kolkata" );
ZonedDateTime zdt = instant.atZone( z );
When taking a zoned date-time back to the database, extract an Instant.
Instant instant = zdt.toInstant();
Be aware that for any given moment, the date as well as the time-of-day varies around the globe by time zone. So if an exact moment matters, such as when a contract expires, beware of using a date-only value. Either use a date-time value for the exact moment, or store the intended time zone alongside the date-only so the exact moment can be calculated later.
LocalDate ld = LocalDate.of( 2016, 1 , 1 );
// Determine the first moment of 2016-01-01 as it happens in Kolkata.
ZonedDateTime zdt = ld.atStartOfDay( ZoneId.of( "Asia/Kolkata" ) );
Instant instant = zdt.toInstant(); // Adjust to UTC and store.
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, & java.text.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 ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
- See How to use….
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.
