[java] Converting between java.time.LocalDateTime and java.util.Date

Java 8 has a completely new API for date and time. One of the most useful classes in this API is LocalDateTime, for holding a timezone-independent date-with-time value.

There are probably millions of lines of code using the legacy class java.util.Date for this purpose. As such, when interfacing old and new code there will be a need for converting between the two. As there seems to be no direct methods for accomplishing this, how can it be done?

Here is what I came up with ( and like all Date Time conundrums it is probably going to be disproved based on some weird timezone-leapyear-daylight adjustment :D )

Round-tripping: Date <<->> LocalDateTime

Given: Date date = [some date]

(1) LocalDateTime << Instant<< Date

    Instant instant = Instant.ofEpochMilli(date.getTime());
    LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);

(2) Date << Instant << LocalDateTime

    Instant instant = ldt.toInstant(ZoneOffset.UTC);
    Date date = Date.from(instant);

Example:

Given:

Date date = new Date();
System.out.println(date + " long: " + date.getTime());

(1) LocalDateTime << Instant<< Date:

Create Instant from Date:

Instant instant = Instant.ofEpochMilli(date.getTime());
System.out.println("Instant from Date:\n" + instant);

Create Date from Instant (not necessary,but for illustration):

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

Create LocalDateTime from Instant

LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);
System.out.println("LocalDateTime from Instant:\n" + ldt);

(2) Date << Instant << LocalDateTime

Create Instant from LocalDateTime:

instant = ldt.toInstant(ZoneOffset.UTC);
System.out.println("Instant from LocalDateTime:\n" + instant);

Create Date from Instant:

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

The output is:

Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

Instant from Date:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

LocalDateTime from Instant:
2013-11-01T14:13:04.574

Instant from LocalDateTime:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

the following seems to work when converting from new API LocalDateTime into java.util.date:

Date.from(ZonedDateTime.of({time as LocalDateTime}, ZoneId.systemDefault()).toInstant());

the reverse conversion can be (hopefully) achieved similar way...

hope it helps...

Much more convenient way if you are sure you need a default timezone :

Date d = java.sql.Timestamp.valueOf( myLocalDateTime );

Everything is here : http://blog.progs.be/542/date-to-java-time

The answer with "round-tripping" is not exact : when you do

LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);

if your system timezone is not UTC/GMT, you change the time !

The fastest way for LocalDateTime -> Date is:

Date.from(ldt.toInstant(ZoneOffset.UTC))

If you are on android and using threetenbp you can use DateTimeUtils instead.

ex:

Date date = DateTimeUtils.toDate(localDateTime.atZone(ZoneId.systemDefault()).toInstant());

you can't use Date.from since it's only supported on api 26+

I'm not sure if this is the simplest or best way, or if there are any pitfalls, but it works:

static public LocalDateTime toLdt(Date date) {
    GregorianCalendar cal = new GregorianCalendar();
    cal.setTime(date);
    ZonedDateTime zdt = cal.toZonedDateTime();
    return zdt.toLocalDateTime();
}

static public Date fromLdt(LocalDateTime ldt) {
    ZonedDateTime zdt = ZonedDateTime.of(ldt, ZoneId.systemDefault());
    GregorianCalendar cal = GregorianCalendar.from(zdt);
    return cal.getTime();
}

I think below approach will solve the conversion without taking time-zone into consideration. Please comment if it has any pitfalls.

    LocalDateTime datetime //input
    public static final DateTimeFormatter yyyyMMddHHmmss_DATE_FORMAT = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    String formatDateTime = datetime.format(yyyyMMddHHmmss_DATE_FORMAT);
    Date outputDate = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(formatDateTime); //output