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Converting Date Time in Java

Converting Date Time in Java

Java 8 is awesome because it introduces lots of good features including the new Date and Time APIs. Now a new challenge follows. How to work with the old classes together with the new ones?

Terminology

  • Date Time vs Timestamp
    • Date Time doesn’t have time zone info
    • Timestamp has time zone info

  • UTC: coordinated universal time

  • Unix epoch time:
    • the number of seconds that have elapsed since January 1, 1970 at midnight UTC time minus the leap seconds
    • at midnight of January 1, 1970, Unix time was 0
    • The Unix epoch is also called Unix time, POSIX time, or Unix timestamp
    • Year 2038 problem:
      • On system that Unix time is as a signed 32-bit number, the representation will end at 3:14:08 on 19 January 2038 UTC
      • In some newer operating systems, it’s been widened to 64-bit, which you don’t need to worry about the ending.

  • DST: Daylight Saving Time

  • ISO 8601: It applies to representations and formats of dates in the Gregorian (and potentially proleptic Gregorian) calendar, of times based on the 24-hour timekeeping system (with optional UTC offset), of time intervals, and combinations thereof. In my opinion, ISO 8601 is clearly superior to other date formats when it comes to international communication.

Archeology

Java Date since Java 1.0

  • java.util.Date in Java 1.0
  • It has no concept of time zone
  • It doesn’t represent a date, but a point in time with millisecond precision
  • It’s intended to reflect UTC, it may not do so exactly, depending on the host environment of the Java Virtual Machine
  • The class Date represents a specific instant in time, with millisecond precision. So, The new Java 8 java.time.Instant is the equivalent class to the classic java.util.Date.
  • It represents the number of seconds passed since the Unix epoch time
  • Years start from 1900
  • months are zero-index based
  • the toString() method is quite misleading because it includes the JVM’s default time zone

java.sql.Date since Java 1.1

  • It extends the Date class

      package java.sql;
  public class Date extends java.util.Date {/**/}
  • A thin wrapper around java.util.Date that allows the JDBC API to identify this as an SQL TIMESTAMP value
  • It is a composite of a java.util.Date and a separate nanoseconds value
  • Use java.sql only for drivers before JDBC 4.2

Timestamp timestamp1 = new Timestamp(System.currentTimeMillis());
Timestamp timestamp2 = new Timestamp(new Date().getTime());

java.sql.Date sqlDate1 = new java.sql.Date(System.currentTimeMillis());  
java.sql.Date sqlDate2 = java.sql.Date.valueOf( "2010-01-31" );

Calendar sicne 1.1

  • java.util.Calendar
  • java.text.DateFormat was introduced to parse the string dates but it’s not thread-safe. If two threads try to parse a date by using the same formatter at the same time, you may receive unpredictable results.
  • months are still zero-index based
  • Calendar class is mutable then there is a thread-safety problem
  • It got rid of the 1900 offset for the year
  • Months also start at index 0
  • it is still problematic to do some calculations as intervals or differences between dates in a simple manner
  • managing zoned dates still gives developers many headaches

Joda-Time

  • A third-party date and time library
  • Java 8 integrates many of the Joda-Time features in the java.time package

Date Time API since Java 1.8

  • java.time package
  • all immutable and thread-safe
  • LocalDate, LocalTime, LocalDateTime
  • Instant
  • TemporalAmount: Duration, Period
  • DateTimeFormatter, DateTimeFormatterBuilder
  • Time Zones: ZoneID, ZoneOffset, OffsetDateTime, OffsetTime

Java 8 Date Time API Details

LocalDate, LocalTime, LocalDateTime

  • Don’t have time zone info
  • Human readable
LocalDate.now(); //get the current date
LocalDate.now(ZoneId.of("Europe/Rome")); //get the current date in a specific zone
LocalDate.of(2020,06,10); //get a date from int values
LocalDate.parse("2020-06-10"); //get a date from string

LocalTime.now();//get the current time
LocalTime.now(ZoneId.of("Europe/Rome"));//get the current time in a specific zone
LocalTime.of(8, 30, 15, 10);//get an instance for 8h 30m 15s 10ns
LocalTime.parse("08:30:15.12345");

LocalDateTime.now();//get the current datetime
LocalDateTime.parse("2020-06-10T08:15:30");//get a datetime parsing the string

Instant

  • Comparing to the above classes, Instant is more like a concept for machine
  • From a machine’s point of view, the most natural format to model time is a single large number representing a point on a continuous timeline
  • This approach is used by the new java.time.Instant class, which represents the number of seconds passed since the Unix epoch time, set by convention to midnight of January 1, 1970 UTC, yes, always UTC
  • the Instant class supports nanosecond precision
  • It supposed not to be with human-readable classes
  • However, it works with Duration and Period classes

Duration, Period

  • All the classes above implement the Temporal interface, which defines how to read and manipulate the values of an object modelling a generic point in time
  • Duration is the time tween two temporal objects
  • It makes sense to human-readable time classes (LocalTime, LocalDateTime) as well as machine-sense classes (Instant). So there is a duration between each category of them.
  • However, you cannot either use LocalDate, because it doesn’t represent time, nor mix of them. DateTimeException is what you’ll get.
  • For the time represents an amount of time in terms of years, months, and days, you can use the Period class
// between
Duration d1 = Duration.between(time1, time2);
Duration d1 = Duration.between(dateTime1, dateTime2);
Duration d2 = Duration.between(instant1, instant2);
Period tenDays = Period.between(LocalDate.of(2001, 9, 11), LocalDate.of(2046, 9, 21));

// create instances
Duration threeMinutes = Duration.ofMinutes(3);
Period tenDays = Period.ofDays(10);

var start = Instant.now();
Thread.sleep(2048);
var duration = Duration.between(start,Instant.now());
System.out.printf("%d seconds and %d ms",duration.toSeconds(),duration.toMillisPart());

The Temporal Package

  • It’s for date and time calculations in low level
  • TemporalAdjusters allow you to manipulate a date in a more complex way than changing one of its values, and you can define and use your own custom date transformations.
// implement your own TemporalAdjusters

Printing and parsing date-time objects

In comparison with the old java.util.DateFormat class, all the DateTimeFormatter instances are thread-safe. Therefore, you can create singleton formatters like the ones defined by the DateTimeFormatter constants and share them among multiple threads.

LocalDate date = LocalDate.of(2046, 3, 18);
String s1 = date.format(DateTimeFormatter.BASIC_ISO_DATE); // 20460318
String s2 = date.format(DateTimeFormatter.ISO_LOCAL_DATE); // 2046-03-18

LocalDate date1 = LocalDate.parse("20140318", DateTimeFormatter.BASIC_ISO_DATE);
LocalDate date2 = LocalDate.parse("2014-03-18", DateTimeFormatter.ISO_LOCAL_DATE);

// creating a DateTimeFormatter from a pattern
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy");
LocalDate date1 = LocalDate.of(2014, 3, 18);
String formattedDate = date1.format(formatter);
LocalDate date2 = LocalDate.parse(formattedDate, formatter);

// todo DateTimeFormatterBuilder

Java 8 with Time Zones using ZoneId

  • A specific ZoneId is identified by a region ID, as in this example:

      ZoneId romeZone = ZoneId.of("Europe/Rome");

  • All the region IDs are in the format “{area}/{city}”, and the set of available locations is the one supplied by the Internet Assigned Numbers Authority (IANA) Time Zone Database.

  • You can also convert an old TimeZone object to a ZoneId by using the new method toZoneId:

      ZoneId zoneId = TimeZone.getDefault().toZoneId();

  • When you have a ZoneId object, you can combine it with a LocalDate, a LocalDateTime, or an Instant to transform it into ZonedDateTime instances, which represent points in time relative to the specified time zone. LocalDate + LocalTime + ZoneId = ZonedDateTime

      LocalDate date = LocalDate.of(2014, Month.MARCH, 18);
  ZonedDateTime zdt1 = date.atStartOfDay(romeZone); // LocalDate + LocalTime (atStartOfDay) + ZoneId

  LocalDateTime dateTime = LocalDateTime.of(2014, Month.MARCH, 18, 13, 45);
  ZonedDateTime zdt2 = dateTime.atZone(romeZone); // LocalDateTime + ZoneId

  Instant instant = Instant.now();
  ZonedDateTime zdt3 = instant.atZone(romeZone);

Java 8 with Time Zones using Offset from UTC

  • Another common way to express a time zone is to use a fixed offset from UTC/Greenwich.
  • ZoneOffset is a subclass of ZoneId, so it should be able to replace ZoneId

  • ZoneOffset represents the difference between a time and the zero meridian of Greenwich, London, as follows:

      ZoneOffset newYorkOffset = ZoneOffset.of("-05:00"); // New York is five hours behind London
  • A ZoneOffset defined this way doesn’t have any Daylight Saving Time management, and for this reason, it isn’t suggested in the majority of cases

Converting Date and Java 8 Date Times

  • Sometimes, you have to work with legacy code and deal with Date class
  • Instant is kinda the middle layer between the old and the new, because it has a similar meaning as the old Date
  • You can also convert a LocalDateTime to an Instant by using a ZoneId and other around

//todo timestamp?

```java
// localDateTime to instant
LocalDateTime dateTime = LocalDateTime.of(2014, Month.MARCH, 18, 13, 45);
Instant instantFromDateTime = dateTime.toInstant(romeZone);

// instant to localDateTime
Instant instant = Instant.now();
LocalDateTime timeFromInstant = LocalDateTime.ofInstant(instant, romeZone);
```

Here is a look up table might help you to understand how they can convert to and from each other, I borrow this idea from YAWK

Convert to Millis java.util.Date java.sql.Date Instant LocalTime LocalDate LocalDateTime ZonedDateTime OffsetDateTime
Initiate long millis = System.currentTimeMillis(); Date date = new Date(); java.sql.Date sqlDate = new java.sql.Date(new java.util.Date().getTime()); Instant instant = Instant.now(); LocalTime localTime = LocalTime.parse(“08:30:15.12345”); LocalDate localDate = LocalDate.of(2020, 06, 10); LocalDateTime localDateTime = LocalDateTime.now(); ZonedDateTime zonedDateTime = instant.atZone(ZoneId.systemDefault()); OffsetDateTime offsetDateTime = instant.atOffset(ZoneOffset.UTC);
Millis - date.getTime(); sqlDate.getTime(); instant.toEpochMilli(); n/a localDate.atStartOfDay(ZoneId.systemDefault()).toInstant().toEpochMilli() localDateTime.toInstant(ZoneOffset.ofTotalSeconds(0)).toEpochMilli(); zonedDateTime.toEpochSecond(); offsetDateTime.toEpochSecond();
java.util.Date new java.util.Date(millis); - java.util.Date date = sqlDate; Date.from(instant); n/a Date.from(localDate.atStartOfDay(ZoneId.systemDefault()).toInstant()) Date.from(localDateTime.atOffset(ZoneOffset.UTC).toInstant()); Date.from(zonedDateTime.toInstant()); Date.from(offsetDateTime.toInstant());
java.sql.Date new java.sql.Date(millis); new java.sql.Date(new Date().getTime()); - java.sql.Date.from(instant); n/a java.sql.Date.valueOf(localDate); java.sql.Date.valueOf(localDateTime.toLocalDate()); java.sql.Date.valueOf(zonedDateTime.toLocalDate()); java.sql.Date.valueOf(offsetDateTime.toLocalDate());
Instant Instant.ofEpochMilli(millis); date.toInstant(); sqlDate.toInstant(); - n/a localDate.atStartOfDay(ZoneId.systemDefault()).toLocalDate(); localDateTime.atOffset(ZoneOffset.UTC).toInstant(); zonedDateTime.toInstant(); offsetDateTime.toInstant();
LocalTime Instant.ofEpochMilli(millis).atOffset(ZoneOffset.UTC).toLocalTime(); n/a n/a n/a - n/a localDateTime.toLocalTime(); zonedDateTime.toLocalTime(); offsetDateTime.toLocalTime();
LocalDate Instant.ofEpochMilli(millis).atOffset(ZoneOffset.UTC).toLocalDate(); date.toInstant().atZone(ZoneId.systemDefault()).toLocalDate(); sqlDate.toLocalDate(); instant.atZone(ZoneId.systemDefault()).toLocalDate(); n/a - localDateTime.toLocalDate(); zonedDateTime.toLocalDate(); offsetDateTime.toLocalDate();
LocalDateTime Instant.ofEpochMilli(millis).atOffset(ZoneOffset.UTC).toLocalDateTime(); date.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime(); sqlDate.toInstant().atOffset(ZoneOffset.UTC).toLocalDateTime(); instant.atZone(ZoneId.systemDefault()).toLocalDateTime(); n/a localDate.atStartOfDay().toLocalTime(); - zonedDateTime.toLocalDateTime(); offsetDateTime.toLocalDateTime()
ZonedDateTime Instant.ofEpochMilli(millis).atZone(ZoneId.systemDefault()); date.toInstant().atZone(ZoneId.systemDefault()); sqlDate.toInstant().atZone(ZoneId.systemDefault()); instant.atZone(ZoneId.of(“Europe/Rome”)); n/a localDate.atStartOfDay(ZoneId.systemDefault()) localDateTime.atZone(ZoneId.of(“Europe/Rome”)); - offsetDateTime.atZoneSameInstant(ZoneId.of(“Europe/Rome”));
OffsetDateTime Instant.ofEpochMilli(millis).atOffset(ZoneOffset.UTC) date.toInstant().atOffset(ZoneOffset.UTC); sqlDate.toInstant().atOffset(ZoneOffset.UTC); instant.atOffset(ZoneOffset.UTC); n/a localDate.atTime(OffsetTime.now(ZoneId.systemDefault())); localDateTime.atOffset(ZoneOffset.UTC); zonedDateTime.toOffsetDateTime(); -

Convert between Date and Java 8 Date Time

  1. convert java.util.Date to java.time.LocalDate

     Date date = new Date(); // instant is the key
 LocalDate localDate = date.toInstant().atZone(ZoneId.systemDefault()).toLocalDate();
 LocalDateTime localDateTime = date.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime();
 ZonedDateTime zonedDateTime = date.toInstant().atZone(ZoneId.systemDefault());
 OffsetDateTime offsetDateTime = date.toInstant().atOffset(ZoneOffset.UTC);

  2. convert java.time.LocalDate to java.util.Date

     ZoneId defaultZoneId = ZoneId.systemDefault();

 LocalDate localDate = LocalDate.of(2046, 8, 19);
 Date date = java.util.Date.from(
     localDate // `LocalDate` class represents a date-only, without time-of-day and without time zone nor offset-from-UTC.
     .atStartOfDay( // Let java.time determine the first moment of the day on that date in that zone. Never assume the day starts at 00:00:00.
         defaultZoneId // Specify time zone using proper name in `continent/region` format, never 3-4 letter pseudo-zones such as “PST”, “CST”, “IST”.
         ) // Produce a `ZonedDateTime` object.
         .toInstant()); // Extract an `Instant` object, a moment always in UTC.

 LocalDateTime localDateTime = LocalDateTime.of(2046,8,19,21,46,31);
 Date date2 = Date.from(localDateTime.atZone(defaultZoneId).toInstant());

 ZonedDateTime zonedDateTime = localDateTime.atZone(defaultZoneId);
 Date date3 = Date.from(zonedDateTime.toInstant());

  3. Convert ZonedDateTime to sql.Timestamp

     java.sql.Timestamp ts = java.sql.Timestamp.from(zdt.toInstant());

  4. Implement your DateUtiles.class

     public class DateUtils {

     public static Date asDate(LocalDate localDate) {
         return Date.from(localDate.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant());
     }

     public static Date asDate(LocalDateTime localDateTime) {
         return Date.from(localDateTime.atZone(ZoneId.systemDefault()).toInstant());
     }

     public static LocalDate asLocalDate(Date date) {
         return Instant.ofEpochMilli(date.getTime()).atZone(ZoneId.systemDefault()).toLocalDate();
     }

     public static LocalDateTime asLocalDateTime(Date date) {
         return Instant.ofEpochMilli(date.getTime()).atZone(ZoneId.systemDefault()).toLocalDateTime();
     }
 }

Conver between formatted String and Date Time

Two main ways:

java.text.SimpleDateFormat is since Java 1.1 java.time.format.DateTimeFormatter is since Java 1.8 Both of them throw DateTimeParseException, which extends RuntimeException

  1. Old way, use SimpleDateFormat

     SimpleDateFormat sdf = new SimpleDateFormat("dd MMM yyyy hh:mm:ss");

 // String to Date
 String dateString = "2019-05-23 00:00:00.0";
 Date date = sdf.parse(dateString);

 // Date to String
 Date today = new Date();
 String date = sdf.format(today);

  2. Java 8, DateTimeFormatter

     DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd MMM yyyy hh:mm:ss");

 // String to Date
 String dateString = "2046-03-21 00:00:00.0";
 LocalDateTime localDateTime = LocalDateTime.parse(dateString, formatter);

 // Date to String
 LocalDateTime localDateTimeNow = LocalDateTime.now();
 String dateTimeNow = formatter.format(localDateTimeNow);

Get date time fields value

  1. The WRONG way

     int year = new Date().getYear();
    • @Deprecated
    • Returns the year represented by this date, minus 1900.

  2. The Calendar API

     Calendar calendar = Calendar.getInstance();
 calendar.setTime(new Date());
 int year = calendar.get(Calendar.YEAR);
 int month = calendar.get(Calendar.MONTH);
 int day = calendar.get(Calendar.DAY_OF_MONTH);
 System.out.println(year);
 System.out.println(month); //Month starts from 0 (January)
 System.out.println(day);

  3. SimpleDateFormat

     Date date = new Date();
 int year = Integer.parseInt(new SimpleDateFormat("yyyy").format(date));
 System.out.println(year); //2046

  4. Java 8 LocalDateTime API

     int year = LocalDate.now().getYear();
 Month month = LocalDate.now().getMonth();
 int monthValue = LocalDate.now().getMonth().getValue();
 int dayOfMonth = LocalDate.now().getDayOfMonth();

Converting table

Based an idea from yawk, here is a converting that easy to look up:

How to persist Date Time

0. What do you want to persist?

  1. Date-only data, e.g., birth day, should not have time and time zone
  2. scheduling future events, e.g., flight ticket

1. Save UTC time without time zone in DB

2. Save the wall time with time zone

Instead of saving the time in UTC along with the time zone, developers can save what the user expects us to save: the wall time. Ie. what the clock on the wall will say. We also save the timezone (Santiago/Chile). This way we can convert back to UTC or any other timezone.

see https://stackoverflow.com/questions/2532729/daylight-saving-time-and-time-zone-best-practices

3. Save a numeric value, using Unix time

  • If you require higher precision, use milliseconds instead
  • This value should always be based on UTC, without any time zone adjustment
  • Not easy to read

Persist Date Time/Timestamp in Postgres

The PostgreSQL™ JDBC driver implements native support for the Java 8 Date and Time API (JSR-310) using JDBC 4.2. ZonedDateTime, Instant and OffsetTime / TIME [ WITHOUT TIMEZONE ] are not supported. Also, note that all OffsetDateTime instance will have to be in UTC (have offset 0). This is because the backend stores them as UTC.

References

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