SeaJUG talk with a few puzzles around Joda Time, java.time, and timestamps in general.

1. This thing all things devours;
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king*, ruins town,
And beats mountain down.
J.R.R. Tolkien
* and software developer alike

2.  2010-08-03T03:30Z
a. sunrise
b. sunset
c. could be either
d. that's no sun!

3.  2010-08-03T03:30Z
a. sunrise
b. sunset
c. could be either
d. that's no sun!

4.  2010-08-03T03:30Z  49.788, -122.987
a. resolve using nearest city
b. resolve using tz_world shapefile
c. resolve with Google Maps API
d. store the time zone offset

5.  2010-08-03T03:30Z  49.788, -122.987
a. resolve using nearest city
b. resolve using tz_world shapefile
c. resolve with Google Maps API
d. store the time zone offset

6.  2010-08-03T03:30Z  49.788, -122.987
a. resolve using nearest city
b. resolve using tz_world shapefile
c. resolve with Google Maps API
d. store the time zone offset

7.  2010-08-03T03:30Z  49.788, -122.987
a. resolve using nearest city
b. resolve using tz_world shapefile
c. resolve with Google Maps API
d. store the time zone offset

8. import org.joda.time.DateTime;
DateTime time = new DateTime("2010-08-02T20:30:00-07:00");
assertEquals(20, time.getHourOfDay());
a. passes
b. fails
c. throws an exception
d. it depends

9. import org.joda.time.DateTime;
DateTime time = new DateTime("2010-08-02T20:30:00-07:00");
assertEquals(20, time.getHourOfDay());
a. passes
b. fails
c. throws an exception
d. it depends

10. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
DateTimeZone zone = DateTimeZone.forOffsetHours(-7);
DateTime time = new DateTime("2010-08-02T20:30:00-07:00", zone);
assertEquals(20, time.getHourOfDay());
Solution #1
Pass an explicit zone to the constructor.

11. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.format.DateTimeFormatter;
import org.joda.time.format.ISODateTimeFormat;
DateTimeFormatter fmt = ISODateTimeFormat
.dateTimeNoMillis().withOffsetParsed();
DateTime time = DateTime.parse("2010-08-02T20:30:00-07:00", fmt);
assertEquals(20, time.getHourOfDay());
Solution #2
Use a custom formatter.

12. import java.time.OffsetDateTime;
OffsetDateTime time =
OffsetDateTime.parse("2010-08-02T20:30:00-07:00");
assertEquals(20, time.getHourOfDay());
Solution #3
Use java.time.OffsetDateTime

13.  9 hours
 2014-03-08T20:00-08:00
a. 5am
b. 6am
c. too early!
d. it depends
When did I get up?

14. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Duration;
DateTimeZone zone = DateTimeZone.forOffsetHours(-8);
DateTime time = new DateTime("2014-03-08T20:00:00", zone);
Duration d = Duration.standardHours(9);
assertEquals(6, time.plus(d).getHourOfDay());
a. passes
b. fails
c. throws an exception
d. it depends

15. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Duration;
DateTimeZone zone = DateTimeZone.forOffsetHours(-8);
DateTime time = new DateTime("2014-03-08T20:00:00", zone);
Duration d = Duration.standardHours(9);
assertEquals(6, time.plus(d).getHourOfDay());
a. passes
b. fails
c. throws an exception
d. it depends

16. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Duration;
DateTimeZone zone = DateTimeZone.forID("America/Los_Angeles");
DateTime time = new DateTime("2014-03-08T20:00:00", zone);
Duration d = Duration.standardHours(9);
assertEquals(6, time.plus(d).getHourOfDay());
Don't do math without time zone IDs!

17. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Duration;
import org.joda.time.Period;
DateTimeZone zone = DateTimeZone.forID("America/Los_Angeles");
DateTime time = new DateTime("2014-03-08T20:00:00", zone);
Duration d = Duration.standardDays(1);
Period p = Period.days(1);
assertEquals(time.plus(d), time.plus(p));
a. passes
b. fails
c. throws an exception
d. it depends

18. import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.Duration;
import org.joda.time.Period;
DateTimeZone zone = DateTimeZone.forID("America/Los_Angeles");
DateTime time = new DateTime("2014-03-08T20:00:00", zone);
Duration d = Duration.standardDays(1);
Period p = Period.days(1);
assertEquals(time.plus(d), time.plus(p));
a. passes
b. fails
c. throws an exception
d. it depends

19. import java.time.DateTime;
import java.time.DateTimeZone;
import java.time.Duration;
import java.time.Period;
ZonedDateTime time = ZonedDateTime
.parse("2014-03-08T20:00:00-08:00[America/Los_Angeles]");
Duration d = Duration.ofDays(1);
Period p = Period.ofDays(1);
assertEquals(time.plus(d), time.plus(p));
a. passes
b. fails
c. throws an exception
d. it depends

20. import org.joda.time.DateTime;
import org.joda.time.Duration;
DateTime begin = new DateTime();
// watch me run 100 m
DateTime end = new DateTime();
Duration d = new Duration(begin, end);
assertFalse(d.isShorterThan(Duration.millis(9580)));
a. never fails
b. usually fails
c. always fails
d. TimeOutException

21. import org.joda.time.DateTime;
import org.joda.time.Duration;
DateTime begin = new DateTime();
// watch me run 100 m
DateTime end = new DateTime();
Duration d = new Duration(begin, end);
assertFalse(d.isShorterThan(Duration.millis(9580)));
a. never fails
b. usually fails
c. always fails
d. TimeOutException

22. import java.util.concurrent.TimeUnit;
import org.joda.time.Duration;
long begin = System.nanoTime();
// watch me run 100 m
long end = System.nanoTime();
Duration d = Duration.millis(
TimeUnit.NANOSECONDS.toMillis(end – begin));
assertFalse(d.isShorterThan(Duration.millis(9580)));
Don't rely on System.currentTimeMillis(),
and store an explicit duration.

23. import org.joda.time.*;
import org.joda.time.Interval;
import org.joda.time.LocalDate;
int steps = 6000;
LocalDate date = LocalDate.parse("2014-11-02");
DateTimeZone zone = DateTimeZone.forID("America/Los_Angeles");
Interval interval = date.toInterval(zone);
long hours = interval.toDuration().getStandardHours();
assertEquals(240, steps / hours);
a. passes
b. fails
c. exception
d. it depends

24. import org.joda.time.*;
import org.joda.time.Interval;
import org.joda.time.LocalDate;
int steps = 6000;
LocalDate date = LocalDate.parse("2014-11-02");
DateTimeZone zone = DateTimeZone.forID("America/Los_Angeles");
Interval interval = date.toInterval(zone);
long hours = interval.toDuration().getStandardHours();
assertEquals(240, steps / hours);
a. passes
b. fails
c. exception
d. it depends

25. Need to store
a timestamp
Is it a range?
no
Does
local time
matter?
Elapsed
time must be
accurate?
yes yes
Need to
add or subtract
time?
Store duration
no no
Store with
zone offset
Store with
zone ID
Store as UTC
no
yes yes
for begin and end
Store interval

26. Illustration by John Tenniel, Alice in Wonderland
Start at the beginning,
and go on til you come to the end:
then stop.