How to get UTC date of a date?

Question

Below is a Java POJO in my application:

public class Appointment {

    private Date appointmentStartDatetime;

    private String timezone;

    //...
}

Example values for these 2 properties are:

2021-10-06 05:30:00
"America/New_York"

appointmentStartDatetime represents the appointment start time in its declared timezone - i.e. the appointment starts at 5.30 in New York.

I want to convert this to a UTC date.

Is the below correct in order to do so?

 public static Date dateToUTC(Date date, String timezoneId){

        Calendar calendar = Calendar.getInstance();
        TimeZone timeZone = TimeZone.getTimeZone(timezoneId);
        calendar.setTimeZone(timeZone);

        return new Date(date.getTime() - calendar.getTimeZone().getOffset(date.getTime()));
    }

The method would be called as follows:

Date utcDate = dateToUTC(appointment.getEventStartDatetime(),appointment.getTimezone())

Answer 1

java.time

To avoid confusion use ZonedDateTime from java.time, the modern Java date and time API, for your date and time with time zone.

public class Appointment {

    private ZonedDateTime appointmentStartDateTime;
    
    // Constructor, etc.

    public Instant getUtcDateTime() {
        return appointmentStartDateTime.toInstant();
    }
    
}

Try it out:

        Appointment myAppointment = new Appointment(
                ZonedDateTime.of(2021, 1, 10, 5, 30, 0, 0,
                        ZoneId.of("America/New_York")));
        
        System.out.println(myAppointment.getUtcDateTime());

Output is:

2021-01-10T10:30:00Z

A ZonedDateTime is a date and time in a time zone. So you need only this one field instead of the two you had, appointmentStartDatetime and timezone. An Instant is a point in time independent of time zone. It prints in UTC, denoted by the trailing Z of the output above. And the conversion from ZonedDateTime to Instant is a matter of a simple method call.

Link

Oracle tutorial: Date Time explaining how to use java.time.

Answer 2

The java.util.Date object is not a real date-time object like the modern date-time types; rather, it represents the number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT (or UTC). When you print an object of java.util.Date, its toString method returns the date-time in the JVM's timezone, calculated from this milliseconds value. If you need to print the date-time in a different timezone, you will need to set the timezone to SimpleDateFormat and obtain the formatted string from it.

Note that the date-time API of java.util and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern date-time API.

• For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7.
• If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.

Using the modern date-time API:

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

class Appointment {
    private LocalDateTime appointmentStartDatetime;
    private String timezone;

    public Appointment(LocalDateTime appointmentStartDatetime, String timezone) {
        this.appointmentStartDatetime = appointmentStartDatetime;
        this.timezone = timezone;
    }

    public LocalDateTime getAppointmentInLocalDateTime() {
        return appointmentStartDatetime;
    }

    public Instant getAppointmentInUTC() {
        return appointmentStartDatetime.toInstant(ZoneOffset.UTC);
    }

    public ZonedDateTime getAppointmentInZonedDateTime() {
        return appointmentStartDatetime.atZone(ZoneId.of(timezone));
    }

    public ZonedDateTime getAppointmentInZdtAnotherTimeZone(String timezone) {
        return getAppointmentInZonedDateTime().withZoneSameInstant(ZoneId.of(timezone));
    }
    // ...
}

public class Main {
    public static void main(String[] args) {
        Appointment appointment = new Appointment(
                LocalDateTime.of(LocalDate.of(2020, Month.FEBRUARY, 15), LocalTime.of(10, 30)), "America/New_York");

        System.out.println(appointment.getAppointmentInLocalDateTime());
        System.out.println(appointment.getAppointmentInUTC());
        System.out.println(appointment.getAppointmentInZonedDateTime());
        System.out.println(appointment.getAppointmentInZdtAnotherTimeZone("Asia/Calcutta"));
    }
}

Output:

2020-02-15T10:30
2020-02-15T10:30:00Z
2020-02-15T10:30-05:00[America/New_York]
2020-02-15T21:00+05:30[Asia/Calcutta]

1. An Instant represents an instantaneous point on the time-line.
2. A ZonedDateTime holds state equivalent to three separate objects, a LocalDateTime, a ZoneId and the resolved ZoneOffset. The function, ZonedDateTime#withZoneSameInstant returns a copy of this date-time with a different time-zone, retaining the instant.

Given below is an overview of the Java SE 8 date-time types:

Learn more about the modern date-time API from Trail: Date Time.