[Java] Date / time operations

Date / time manipulation

• ** Use Date-Time-API ** (Java 8 or later)

Get current time

• ** Local Date ** is the date and time
• ** Local Time ** is the time

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.OffsetDateTime;
import java.time.ZonedDateTime;

public class Main {
  //Get the current date and time
  public static void main(String[] args) {
    System.out.println(LocalDateTime.now());  //2020-10-31T07:49:16.502455881
    System.out.println(OffsetDateTime.now()); //2020-10-31T07:49:16.503291965Z
    System.out.println(ZonedDateTime.now());  //020-10-31T07:49:16.504062367Z[Etc/UTC]
    System.out.println(LocalDate.now());      //2020-10-31
    System.out.println(LocalTime.now());      //07:49:16.504349859
  }
}

Get a specific date and time

• ** LocalDateTime.of method **

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.Month;
import java.time.OffsetDateTime;
import java.time.OffsetTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;


public class Main {
  //Get a specific date and time
  public static void main(String[] args) {
    //Generate date and time from date / hour / minute / second nanosecond
    var ldt1 = LocalDateTime.of(2019, 1, 10, 10, 20, 30, 513);
    var ldt2 = LocalDateTime.of(2019, Month.JANUARY, 10, 10, 20, 30);
    //var ldt3 = LocalDateTime.of(2019, 1, 40, 10, 20, 30); //Error because it is out of range
    System.out.println(ldt1); //2019-01-10T10:20:30.000000513
    System.out.println(ldt2); //2019-01-10T10:20:30.000000513

    //Generate only date / time
    var ld = LocalDate.of(2019, 1, 10);
    System.out.println(ld);   //2019-01-10
    var lt = LocalTime.of(10, 20, 30);
    System.out.println(lt);  //10:20:30
    var ldt4 = LocalDateTime.of(ld, lt);
    System.out.println(ldt4); //2019-01-10T10:20:30

    //Time zone offset value(ZoneOffset object)
    var odt = OffsetDateTime.of(2019, 1, 10, 10, 20, 30, 999, ZoneOffset.ofHours(9));
    System.out.println(odt); //2019-01-10T10:20:30.000000999+09:00

    var ot = OffsetTime.of(10, 20, 30, 999, ZoneOffset.ofHours(9));
    System.out.println(ot); //10:20:30.000000999+09:00

    var zdt = ZonedDateTime.of(2019, 1, 10, 10, 20, 30, 999, ZoneId.of("Asia/Tokyo"));
    System.out.println(zdt); //2019-01-10T10:20:30.000000999+09:00[Asia/Tokyo]
  }
}

Convert datetime string

• ** parse method **
• If you specify formatter, you can get the value parsed in the specified format.

import java.time.LocalDate;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;


public class Main {
  //Convert from date / time string
  public static void main(String[] args) {
    //ISO date
    System.out.println(LocalDate.parse(
        "2019-01-01", DateTimeFormatter.ISO_DATE)); //2019-01-01
    //Year and days
    System.out.println(LocalDate.parse(
        "2019-123", DateTimeFormatter.ISO_ORDINAL_DATE)); //2019-05-03
    //Year and number of weeks
    System.out.println(LocalDate.parse(
        "2019-W40-5", DateTimeFormatter.ISO_WEEK_DATE)); //2019-10-04
    //Date and time with time zone ID
    System.out.println(ZonedDateTime.parse(
      "2019-01-10T10:20:30.000000999+09:00[Asia/Tokyo]",
      DateTimeFormatter.ISO_DATE_TIME)); //2019-01-10T10:20:30.000000999+09:00[Asia/Tokyo]
  }
}

Compare date and time

• equals / isBefore / isAfter

import java.time.LocalDateTime;
public class Main {
  public static void main(String[] args) {
    var dt1 = LocalDateTime.of(2019, 12, 31, 10, 20, 30);
    var dt2 = LocalDateTime.of(2020, 1, 1, 10, 20, 30);
    System.out.println(dt1.equals(dt2));   //false
    System.out.println(dt1.isBefore(dt2)); //true
    System.out.println(dt1.isAfter(dt2));  //false
  }
}

Get time element

import java.time.LocalDateTime;
import java.time.temporal.ChronoField;

public class Main {
  public static void main(String[] args) {
    var dt = LocalDateTime.of(2019, 1, 10, 10, 20, 30, 123);
    var week = new String[] {"Sunday", "Monday", "Tuesday", "Wednesday","Thursday", "Friday", "Saturday"};
    System.out.println(dt.getYear() + "Year" +dt.getMonthValue() + "Moon" +dt.getDayOfMonth() + "Day" +dt.getDayOfWeek() + " " +dt.getHour() + "Time" +dt.getMinute() + "Minutes"+ dt.getSecond() + "Seconds"+ dt.getNano() + "ナノSeconds");
    System.out.println("The month name is" + dt.getMonth() +"this year" + dt.getDayOfYear() + "Day"); //The month name isJANUARYthis year10Day
    //Get time with Chrono Field
    System.out.println(dt.get(ChronoField.YEAR) + "Year" +dt.get(ChronoField.MONTH_OF_YEAR) + "Moon" +dt.get(ChronoField.DAY_OF_MONTH) + "Day" +week[dt.get(ChronoField.DAY_OF_WEEK) -1] + " "+dt.get(ChronoField.HOUR_OF_DAY) + "Time" +dt.get(ChronoField.MINUTE_OF_HOUR) + "Minutes" +dt.get(ChronoField.SECOND_OF_MINUTE) + "Seconds" +dt.get(ChronoField.NANO_OF_SECOND) + "ナノSeconds");
  }
} //Wednesday, January 10, 2019 10:20:30 123 nanoseconds

Format date and time

• ** Format according to the argument of DateTimeFormatter method **

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.FormatStyle;

public class Main {
  public static void main(String[] args) {
    var dt1 = LocalDateTime.of(2020, 11, 1, 10, 20, 30);
    var dt2 = ZonedDateTime.of(2019, 11, 1, 10, 20, 30, 0, ZoneId.of("Asia/Tokyo"));

    System.out.println(dt1.format(DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL)));      //Sunday, November 1, 2020
    System.out.println(dt2.format(DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG)));  //November 1, 2019 at 10:20:30 AM JST
    System.out.println(dt1.format(DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM)));    //Nov 1, 2020
    System.out.println(dt2.format(DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT))); //11/1/19, 10:20 AM
  }
}

• ** Format to your liking with DateTimeFormatter.ofPattern method **
• The standard format is recommended because it will give you an appropriate format (English, etc.) according to the locale.

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class Main {
  public static void main(String[] args) {
    var dt1 = LocalDateTime.of(2020, 11, 1, 10, 20, 30);
    var dt2 = ZonedDateTime.of(2020, 11, 1, 10, 20, 30, 0, ZoneId.of("Asia/Tokyo"));
    System.out.println(dt1.format(DateTimeFormatter.ofPattern("y.MM.dd H:m:s"))); //2020.11.01 10:20:30
    System.out.println(dt2.format(DateTimeFormatter.ofPattern("Y year L month d day (E) a K hour m minute s second(z)"))); //November 1, 2020 (Sun) 10:20:30 AM(JST)
  }
}

• ** Generate date from era with JapaneseDate.of method **
• var d = JapaneseDate.of (JapaneseEra.REIWA, 1, 1, 1); is Error

//Japanese Calendar
import java.time.chrono.JapaneseDate;
import java.time.chrono.JapaneseEra;
import java.time.format.DateTimeFormatter;

public class Main {
  public static void main(String[] args) {
    var d = JapaneseDate.of(JapaneseEra.REIWA, 2, 1, 1);
    System.out.println(d); //Japanese Reiwa 2-01-01
    //G is the era
    var df = DateTimeFormatter.ofPattern("Gy year MM month dd day");
    System.out.println(d.format(df)); //Reiwa January 01, 2
  }
}

Date-time difference difference

• Period (date) / Duration (time) class ** between method **
• Period argument needs to be converted to LocalDate once with toLocalDate
• Duration argument is Temporal type: Can accept all instances of LocalDate and ZonedDate

//Days elapsed count
import java.time.Duration;
import java.time.LocalDateTime;
import java.time.Period;

public class Main {
  public static void main(String[] args) {
    var dt1 = LocalDateTime.of(2019, 12, 31, 0, 0, 0);
    var dt2 = LocalDateTime.of(2020, 11, 1, 10, 20, 30);

    var period = Period.between(dt1.toLocalDate(), dt2.toLocalDate());
    System.out.println("Date difference:" +
      period.getYears() + "Year" + period.getMonths() + "months" +
      period.getDays() + "Days"); //日付の差：0年10ヶ月1Days

    var duration = Duration.between(dt1, dt2);
    System.out.println("Time difference:" + duration.toHours() + "time"); //Time difference:7354time
  }
}

Date addition subtraction

• ** plus / minus method **
• ** There are also methods that specialize in time calculation, such as plusYears / plusWeeks **.

//x years later,y days ago
import java.time.LocalDate;
import java.time.temporal.ChronoUnit;
public class Main {
  public static void main(String[] args) {
    var d = LocalDate.of(2020, 11, 1);
    System.out.println(d); //2020-11-01
    //3 years later
    System.out.println(d.plus(3, ChronoUnit.YEARS)); //2023-11-01
    //22 days ago
    System.out.println(d.minus(22, ChronoUnit.DAYS)); //2020-10-10
  }
}

• Pass Period / Duration and add / subtract at date intervals
• Period: ** P **
• P2Y2M = 2 years and 2 months
• Duration: ** P T **
• -P1DT5M = 1 day 5 minutes ago

import java.time.Duration;
import java.time.LocalDateTime;
import java.time.Period;
public class Main {
  public static void main(String[] args) {
    var d = LocalDateTime.of(2020, 11, 1, 10, 20, 30);
    var period = Period.ofYears(3);
    //P<date>T<time>
    var duration = Duration.parse("P22DT1H1M1S"); //22 days 1 hour 1 minute 1 second
    System.out.println(d);  //2020-11-01T10:20:30
    System.out.println(d.plus(period)); //2023-11-01T10:20:30
    System.out.println(d.minus(duration)); //2020-10-10T09:19:29
  }
}

Calendar class (Java 7 or earlier)

• ** Convert from Calendar to Date-Time API with toInstant method **
• toInstant method: Keeps elapsed time from 1 / 1970/1
• ** ofInstant method ** creates objects such as LocalDateTime

//Calendar to Date-Convert to Time API
import java.time.LocalDateTime;
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.Calendar;

public class Main {
  public static void main(String[] args) {
    var cal = Calendar.getInstance();
    var dt1 = LocalDateTime.ofInstant(cal.toInstant(), ZoneId.systemDefault());
    var dt2 = OffsetDateTime.ofInstant(cal.toInstant(), ZoneId.systemDefault());
    var dt3 = ZonedDateTime.ofInstant(cal.toInstant(), ZoneId.systemDefault());
    System.out.println(dt1); //2020-11-01T03:30:12.156
    System.out.println(dt2); //2020-11-01T03:30:12.156Z
    System.out.println(dt3); //2020-11-01T03:30:12.156Z[Etc/UTC]
  }
}

• ** Convert from Date-Time API to Calender **
• Calendar accuracy is in milliseconds, so truncate less than milliseconds
• Receive the generated Instant object with Date.from method ** Date object ** Generate
• ** SetTime method ** to enter a value from Date to Calendar

//Date-Convert from Time API to Calender
import java.time.LocalDateTime;
import java.time.ZoneOffset;
import java.util.Calendar;
import java.util.Date;

public class Main {
  public static void main(String[] args) {
    var dt = LocalDateTime.of(2020, 11, 1, 10, 20, 30, 123456789);
    var d = Date.from(dt.toInstant(ZoneOffset.of("+09:00")));
    //Enter a value from Date to Calendar with the setTime method
    var cal = Calendar.getInstance();
    cal.setTime(d);
    System.out.println(cal); //java.util.GregorianCalendar[time=1604193630123,,(Abbreviation),,AM_PM=0,HOUR=1,HOUR_OF_DAY=1,MINUTE=20,SECOND=30,MILLISECOND=123,ZONE_OFFSET=0,DST_OFFSET=0]
    }
}