[java] How to set time to a date object in java

I created a Date object in Java. When I do so, it shows something like: date=Tue Aug 09 00:00:00 IST 2011. As a result, it appears that my Excel file is lesser by one day (27 feb becomes 26 feb and so on) I think it must be because of time. How can I set it to something like 5:30 pm?

Can you show code which you use for setting date object? Anyway< you can use this code for intialisation of date:

new SimpleDateFormat("yyyy-MM-dd hh:mm:ss").parse("2011-01-01 00:00:00")

If you don't have access to java 8 and the API java.time, here is my simple function to copy the time of one date to another date using the old java.util.Calendar (inspire by Jigar Joshi) :

/**
 * Copy only the time of one date to the date of another date.
 */
public static Date copyTimeToDate(Date date, Date time) {
    Calendar t = Calendar.getInstance();
    t.setTime(time);

    Calendar c = Calendar.getInstance();
    c.setTime(date);
    c.set(Calendar.HOUR_OF_DAY, t.get(Calendar.HOUR_OF_DAY));
    c.set(Calendar.MINUTE, t.get(Calendar.MINUTE));
    c.set(Calendar.SECOND, t.get(Calendar.SECOND));
    c.set(Calendar.MILLISECOND, t.get(Calendar.MILLISECOND));
    return c.getTime();
}

I should like to contribute the modern answer. This involves using java.time, the modern Java date and time API, and not the old Date nor Calendar except where there’s no way to avoid it.

Your issue is very likely really a timezone issue. When it is Tue Aug 09 00:00:00 IST 2011, in time zones west of IST midnight has not yet been reached. It is still Aug 8. If for example your API for putting the date into Excel expects UTC, the date will be the day before the one you intended. I believe the real and good solution is to produce a date-time of 00:00 UTC (or whatever time zone or offset is expected and used at the other end).

    LocalDate yourDate = LocalDate.of(2018, Month.FEBRUARY, 27);
    ZonedDateTime utcDateDime = yourDate.atStartOfDay(ZoneOffset.UTC);
    System.out.println(utcDateDime);

This prints

2018-02-27T00:00Z

Z means UTC (think of it as offset zero from UTC or Zulu time zone). Better still, of course, if you could pass the LocalDate from the first code line to Excel. It doesn’t include time-of-day, so there is no confusion possible. On the other hand, if you need an old-fashioned Date object for that, convert just before handing the Date on:

    Date oldfashionedDate = Date.from(utcDateDime.toInstant());
    System.out.println(oldfashionedDate);

On my computer this prints

Tue Feb 27 01:00:00 CET 2018

Don’t be fooled, it is correct. My time zone (Central European Time) is at offset +01:00 from UTC in February (standard time), so 01:00:00 here is equal to 00:00:00 UTC. It’s just Date.toString() grabbing the JVMs time zone and using it for producing the string.

How can I set it to something like 5:30 pm?

To answer your direct question directly, if you have a ZonedDateTime, OffsetDateTime or LocalDateTime, in all of these cases the following will accomplish what you asked for:

    yourDateTime = yourDateTime.with(LocalTime.of(17, 30));

If yourDateTime was a LocalDateTime of 2018-02-27T00:00, it will now be 2018-02-27T17:30. Similarly for the other types, only they include offset and time zone too as appropriate.

If you only had a date, as in the first snippet above, you can also add time-of-day information to it:

    LocalDate yourDate = LocalDate.of(2018, Month.FEBRUARY, 27);
    LocalDateTime dateTime = yourDate.atTime(LocalTime.of(17, 30));

For most purposes you should prefer to add the time-of-day in a specific time zone, though, for example

    ZonedDateTime dateTime = yourDate.atTime(LocalTime.of(17, 30))
            .atZone(ZoneId.of("Asia/Kolkata"));

This yields 2018-02-27T17:30+05:30[Asia/Kolkata].

Date and Calendar vs java.time

The Date class that you use as well as Calendar and SimpleDateFormat used in the other answers are long outdated, and SimpleDateFormat in particular has proven troublesome. In all cases the modern Java date and time API is so much nicer to work with. Which is why I wanted to provide this answer to an old question that is still being visited.

Link: Oracle Tutorial Date Time, explaining how to use java.time.