datetime - Java Calendar adds a random number of milliseconds?

Question

Hi I have something weird happening. I am simply taking a calendar object, converting it to its individual parts, and putting it back into a calendar (with or without any changes. In this case I make no changes). I mean, this should be cut and paste. I've also tried to create a calendar with calendar = Calendar.getInstance() and set everything manually. calendar.set(Calendar.YEAR, mStartYear); so on. Still gives wrong Calendar objects. I've tried also setting Milliseconds, always seem to have some garbage milliseconds.. But the time one way or another just is completely off. Maybe someone sees the stupid oversight, but I'm stumped.

Also this is an android application, but shouldn't matter for a basic Java library object.

Note Weekview is a datawrapper for one of the libraries I'm using. It has a start and an end calendar.

Here is what the debugger lists as in memory..

mEndDay = 19
mEndHour = 9

mEndMinute = 30
mEndMonth = 8
mEndYear = 2015
mSeekAmount = 0
mStartDay = 18
mStartHour = 23
mStartMinute = 0
mStartMonth = 8
mStartYear = 2015


            Calendar calendarStart = Calendar.getInstance();
            calendarStart.set(mStartYear,mStartMonth,mStartDay,mStartHour,mStartMinute);

            Calendar calendarEnd = Calendar.getInstance();
            calendarEnd.set(mEndYear,mEndMonth,mEndDay,mEndHour,mEndMinute);

I end up with

Start 1442363359161
End 1442363359161

calendarStart = {GregorianCalendar@20968} "java.util.GregorianCalendar[time=?,areFieldsSet=false,lenient=true,zone=America/Denver,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2015,MONTH=8,WEEK_OF_YEAR=38,WEEK_OF_MONTH=3,DAY_OF_MONTH=18,DAY_OF_YEAR=258,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=3,AM_PM=1,HOUR=6,HOUR_OF_DAY=23,MINUTE=0,SECOND=19,MILLISECOND=161,ZONE_OFFSET=-25200000,DST_OFFSET=3600000]"
calendarEnd = {GregorianCalendar@20969} "java.util.GregorianCalendar[time=?,areFieldsSet=false,lenient=true,zone=America/Denver,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2015,MONTH=8,WEEK_OF_YEAR=38,WEEK_OF_MONTH=3,DAY_OF_MONTH=19,DAY_OF_YEAR=258,DAY_OF_WEEK=3,DAY_OF_WEEK_IN_MONTH=3,AM_PM=1,HOUR=6,HOUR_OF_DAY=9,MINUTE=30,SECOND=19,MILLISECOND=161,ZONE_OFFSET=-25200000,DST_OFFSET=3600000]"

EXPECT

Start 1442638800000
End 1442676600000

mEndTime = {GregorianCalendar@20990} "java.util.GregorianCalendar[time=1442676600000,areFieldsSet=true,lenient=true,zone=America/Denver,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2015,MONTH=8,WEEK_OF_YEAR=38,WEEK_OF_MONTH=3,DAY_OF_MONTH=19,DAY_OF_YEAR=262,DAY_OF_WEEK=7,DAY_OF_WEEK_IN_MONTH=3,AM_PM=0,HOUR=9,HOUR_OF_DAY=9,MINUTE=30,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-25200000,DST_OFFSET=3600000]"
mName = {String@20991} "sleep"
mStartTime = {GregorianCalendar@20992} "java.util.GregorianCalendar[time=1442638800000,areFieldsSet=true,lenient=true,zone=America/Denver,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2015,MONTH=8,WEEK_OF_YEAR=38,WEEK_OF_MONTH=3,DAY_OF_MONTH=18,DAY_OF_YEAR=261,DAY_OF_WEEK=6,DAY_OF_WEEK_IN_MONTH=3,AM_PM=1,HOUR=11,HOUR_OF_DAY=23,MINUTE=0,SECOND=0,MILLISECOND=0,ZONE_OFFSET=-25200000,DST_OFFSET=3600000]"

Here is where I set it up initially..

   WeekViewEvent weekViewEvent = dateWrapperParam.getWeekViewEvent();
    Calendar endCalendar = weekViewEvent.getEndTime();
    Calendar startCalendar = weekViewEvent.getStartTime();

    Date endDate = endCalendar.getTime();
    Date startDate = startCalendar.getTime();

    mStartHour = startCalendar.get(Calendar.HOUR_OF_DAY);
    mStartMinute = startCalendar.get(Calendar.MINUTE);
    mStartDay = startCalendar.get(Calendar.DAY_OF_MONTH);
    mStartMonth = startCalendar.get(Calendar.MONTH);
    mStartYear = startCalendar.get(Calendar.YEAR);

    mEndHour = endCalendar.get(Calendar.HOUR_OF_DAY);
    mEndMinute = endCalendar.get(Calendar.MINUTE);
    mEndDay = endCalendar.get(Calendar.DAY_OF_MONTH);
    mEndMonth = endCalendar.get(Calendar.MONTH);
    mEndYear = endCalendar.get(Calendar.YEAR);

Answer 1

In the documentation of Calendar.set, it is said :

Sets the values for the fields YEAR, MONTH, DAY_OF_MONTH, HOUR, MINUTE, and SECOND. Previous values of other fields are retained. If this is not desired, call clear() first.

The reason is that not all fields are set with this method, in you case, you don't have MILLISECOND set. So it keep the value when the instance was created.

The call of Calendar.clear will

Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this Calendar undefined.

A quick example :

    Calendar c = GregorianCalendar.getInstance();
    c.clear();
    c.set(2019, Calendar.NOVEMBER, 03, 16, 15, 03);

    System.out.println(c.getTime());
    System.out.println(c.getTimeInMillis());

Sun Nov 03 16:15:03 CET 2019
1572794103000

Milliseconds being undefined will give 0