如何在Java中设置日期的最小小时数？

Question

I'm trying to get the date and time of the first day of the prior month; specifically as it is January I'm trying to get: Sun Dec 01 00:00:00 EST 2013 . I am using the below code snippet which I have modified from another found here on Stackoverflow while researching this subject; this code snippet will return: Sun Dec 01 12:00:00 EST 2013 . I do not understand why setting the minimum hour for the 1st in fact returns noon.

Calendar aCalendar = Calendar.getInstance();
// add -1 month to current month
aCalendar.add(Calendar.MONTH, -1);
// set DATE to 1, so first date of previous month
//aCalendar.set(Calendar.DATE, 1);                        
aCalendar.set(Calendar.DATE, aCalendar.getActualMinimum(Calendar.DAY_OF_MONTH));
aCalendar.set(Calendar.HOUR, aCalendar.getActualMinimum(Calendar.HOUR));
aCalendar.set(Calendar.MINUTE, aCalendar.getActualMinimum(Calendar.MINUTE));
aCalendar.set(Calendar.SECOND, aCalendar.getActualMinimum(Calendar.SECOND));
Date firstDateOfPreviousMonth = aCalendar.getTime();

If I modify the following line as shown and set 0 I get the same result:

aCalendar.set(Calendar.HOUR, 0);

If I modify it as follows I get 1pm:

aCalendar.set(Calendar.HOUR, 0);

Result: Sun Dec 01 13:00:00 EST 2013

Answer 1

From the Java Docs

HOUR Field number for get and set indicating the hour of the morning or afternoon.

You'll want to use HOUR_OF_DAY instead

HOUR_OF_DAY Field number for get and set indicating the hour of the day.

Or set the AM_PM accordingly...

AM_PM Field number for get and set indicating whether the HOUR is before or after noon.

In any case, take the time to consult the Java Docs

Answer 2

Some example code using Joda-Time 2.3.

Using default time zone, we create a DateTime instance of now (current moment), go back to previous month (in a smart manner, compensating for short month like February), get the first of month by asking for "minimum value", and then get first moment of that first-of-month day.

org.joda.time.DateTime startOfPreviousMonth = new org.joda.time.DateTime().minusMonths(1).dayOfMonth().withMinimumValue().withTimeAtStartOfDay();

Note the method withTimeAtStartOfDay to get the first moment of the day. That moment may or may not be 00:00:00 in local time (because of Daylight Saving Time or other anomalies).

Using explicit time zone (almost always a better practice)…

DateTimeZone timeZone = DateTimeZone.forId( "Europe/Paris" ); // "Asia/Kolkata" etc.
org.joda.time.DateTime startOfPreviousMonth = new org.joda.time.DateTime( timeZone ).minusMonths(1).dayOfMonth().withMinimumValue().withTimeAtStartOfDay();

Get a string…

String string = startOfPreviousMonth; // or startOfPreviousMonth.toString();

To convert back to a java.util.Date instance for communicating with other libraries…

java.util.Date date = startOfPreviousMonth.toDate();