Time

This tutorial shows how to handle time in Orekit.

Basic use

The first example will be devoted to basic use: creating dates, comparing dates and printing dates. We will build a small program that loops between a start and an end date with a small step and print it in two different time scales: UTC and TAI. The loop will be around a leap second introduction to see the offset change in the two time scales.

The complete code for this example can be found in the source tree of the library, in file src/tutorials/fr/cs/examples/time/Time1.java.

As explained in the time section of the library architecture documentation, dates are defined with respect to time scales. We first get the two time scales we need. These scales are singletons, so we do not build them but retrieve the singletons instances as follows:

TimeScale utc = TimeScalesFactory.getUTC();
TimeScale tai = TimeScalesFactory.getTAI();

Once we have the UTC time scale available, we can create the start date from its components with respect to the UTC time scale:

AbsoluteDate start = new AbsoluteDate(2005, 12, 31, 23, 59, 50, utc);

For the end date of our loop, we will use a relative definition instead of an absolute definition. We want the end date to be 20 seconds after the start date.

double duration = 20.0;
AbsoluteDate end = new AbsoluteDate(start, duration);

We then print a header for our output.

System.out.println("        UTC date                  TAI date");

Now comes the real processing: the loop. Since AbsoluteDate instances are immutable, we use a new instance at each iteration. For the first iteration, we simply reuse the start date, and for other iterations we create an instance relative to the previous one.

double step = 0.5;
for (AbsoluteDate date = start;
     date.compareTo(end) < 0;
     date = new AbsoluteDate(date, step)) {
   // loop body
}

Inside the loop body (i.e. replacing the comment in the code snippet above), we print the date in the two different time scales. One important thing to notice is that we use only one instance of AbsoluteDate, but can locate it with respect to several different time scales.

System.out.println(date.toString(utc) + "   " + date.toString(tai));

Since the loop brackets a leap second introduction, we see two different things in the output of this program. First, the middle iterations show the last minute of the day in UTC time scale has more than 60 seconds: we see dates 2005-12-31T23:59:60.000
and 2005-12-31T23:59:60.500. Second, we see the offset between the two time scales grow from 32 seconds at the start of the loop to 33 seconds at the end of the loop: the leap second has been taken into account.

        UTC date                  TAI date
2005-12-31T23:59:50.000   2006-01-01T00:00:22.000
2005-12-31T23:59:50.500   2006-01-01T00:00:22.500
          ... 16 lines deleted here ...
2005-12-31T23:59:59.000   2006-01-01T00:00:31.000
2005-12-31T23:59:59.500   2006-01-01T00:00:31.500
2005-12-31T23:59:60.000   2006-01-01T00:00:32.000
2005-12-31T23:59:60.500   2006-01-01T00:00:32.500
2006-01-01T00:00:00.000   2006-01-01T00:00:33.000
2006-01-01T00:00:00.500   2006-01-01T00:00:33.500
          ... 14 lines deleted here ...
2006-01-01T00:00:08.000   2006-01-01T00:00:41.000
2006-01-01T00:00:08.500   2006-01-01T00:00:41.500