Resetting the time part of a timestamp in Java

Assuming your "timestamp" is a java.util.Date, which is represented as the number of milliseconds since the beginning of the epoch (Jan 1, 1970), you can perform the following arithmetic:

public static Date stripTimePortion(Date timestamp) {
    long msInDay = 1000 * 60 * 60 * 24; // Number of milliseconds in a day
    long msPortion = timestamp.getTime() % msInDay;
    return new Date(timestamp.getTime() - msPortion);
}

If you're using commons lang you can call DateUtils.truncate. Here's the javadoc documentation.

It does the same thing @Alex Miller said to do.

You can go Date->Calendar->set->Date:

Date date = new Date();                      // timestamp now
Calendar cal = Calendar.getInstance();       // get calendar instance
cal.setTime(date);                           // set cal to date
cal.set(Calendar.HOUR_OF_DAY, 0);            // set hour to midnight
cal.set(Calendar.MINUTE, 0);                 // set minute in hour
cal.set(Calendar.SECOND, 0);                 // set second in minute
cal.set(Calendar.MILLISECOND, 0);            // set millis in second
Date zeroedDate = cal.getTime();             // actually computes the new Date

I love Java dates.

Note that if you're using actual java.sql.Timestamps, they have an extra nanos field. Calendar of course, knows nothing of nanos so will blindly ignore it and effectively drop it when creating the zeroedDate at the end, which you could then use to create a new Timetamp object.

I should also note that Calendar is not thread-safe, so don't go thinking you can make that a static single cal instance called from multiple threads to avoid creating new Calendar instances.