test.java

@Test
public void timezoneTest() throws Exception {
    long slotSize = 86400000; // ms -> 1day
    Calendar caldendar = Calendar.getInstance();

    long origCurTime = caldendar.getTimeInMillis();
    caldendar.setTimeZone(TimeZone.getTimeZone("Asia/Seoul"));
    long curTime = caldendar.getTimeInMillis();

    long tzOffset = TimeZone.getDefault().getRawOffset();
    long curTimeWithTimeZone = curTime + tzOffset;
    long offset = curTimeWithTimeZone % slotSize;
    long biTime = curTime - offset - slotSize;

    System.out.println(String.format("orig curTime: %d", origCurTime));
    System.out.println(String.format("curTime: %d", curTime));
    System.out.println(String.format("tzOffset: %d", tzOffset));
    System.out.println(String.format("curTimeWithTimeZone: %d", curTimeWithTimeZone));
    System.out.println(String.format("offset: %d", offset));
    System.out.println(String.format("biTime: %d", biTime));
    System.out.println(String.format("biTime date: " + new Date(biTime) ));
    System.out.println(String.format("orig date: " + caldendar.getTime() ));


    System.out.println("################################################################################");

    ZonedDateTime now = LocalDateTime.now().atZone(ZoneId.of("Asia/Seoul"));
    System.out.println(String.format("orig date: " + now));
    System.out.println(String.format("orig date (millisec): " + now.toInstant().toEpochMilli()));
    ZonedDateTime startOfDayInYesterday = now.minusDays(1)
            .withHour(0)
            .withMinute(0)
            .withSecond(0)
            .withNano(0);

    System.out.println(String.format("biTime date: " + startOfDayInYesterday));
    System.out.println(String.format("biTime date: " +
            startOfDayInYesterday.toInstant().toEpochMilli()));
}

orig curTime: 1456363640552
curTime: 1456363640552
tzOffset: 32400000
curTimeWithTimeZone: 1456396040552
offset: 37640552
biTime: 1456239600000
biTime date: Wed Feb 24 00:00:00 KST 2016
orig date: Thu Feb 25 10:27:20 KST 2016

orig date: 2016-02-25T10:27:20.594+09:00[Asia/Seoul]
orig date (millisec): 1456363640594
biTime date: 2016-02-24T00:00+09:00[Asia/Seoul]
biTime date: 1456239600000