The correct way to set and get hour, minutes, sec

Question

I am trying to get some information out of a database and then using that information to get some statistics.

I want to get statistics based on an interval of hours,

Answer 1

Getting Date-Time From Database

Getting date-time from a database has been addressed in hundreds of answers. Please search StackOverflow. Focus on java.sql.Timestamp.

To address the topic of your Question’s title, read on.

Joda-Time

Far easier if you use either Joda-Time or the java.time package bundled with Java 8 (inspired by Joda-Time). The java.util.Date & .Calendar classes bundled with Java are notoriously troublesome, confusing, and flawed.

Time zone is crucial. Unlike java.util.Date, both Joda-Time and java.time assign a time zone to their date-time objects.

Here is some example code to show multiple ways to set the time-of-day on a Joda-Time 2.5 DateTime object.

DateTimeZone zoneMontreal = DateTimeZone.forID( "America/Montreal" );  // Specify a time zone, or else the JVM's current default time zone will be assigned to your new DateTime objects.
DateTime nowMontreal = DateTime.now( zoneMontreal );  // Current moment.
DateTime startOfDayMontreal = nowMontreal.withTimeAtStartOfDay();  // Set time portion to first moment of the day. Usually that means 00:00:00.000 but not always.
DateTime fourHoursAfterStartOfDayMontreal = startOfDayMontreal.plusHours( 4 ); // You can add or subtract hours, minutes, and so on.
DateTime todayAtThreeInAfternoon = nowMontreal.withTime(15, 0, 0, 0);  // Set a specific time of day.

Converting

If you absolutely need a java.util.Date object, convert from Joda-Time.

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

To go from j.u.Date to Joda-Time, pass the Date object to constructor of Joda-Time DateTime.

Answer 2

Using the java.util.Calendar

Calendar c = Calendar.getInstance();
c.set(Calendar.DATE, 2);
c.set(Calendar.HOUR_OF_DAY, 1);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);
c.set(Calendar.MILLISECOND, 0);

Or use Joda Time http://joda-time.sourceforge.net/.