Core Java

The lepidopterist’s curse: Playing with java.time

Pop quiz: What will be the output of this little program?

public class DateFun {
    public static void main(String[] args) {
        long hours = getHoursOfDay(, ZoneId.systemDefault());
    private static long getHoursOfDay(LocalDate date, ZoneId zoneId) {
        ZonedDateTime startOfDay = date.atStartOfDay(zoneId);
        Duration duration = Duration.between(startOfDay, startOfDay.plusDays(1));
        return duration.toHours();

The answer is, like with most interesting questions, “it depends”. How can it depend? Well, let’s try a few examples:

  • getHoursOfDay(LocalDate.of(2014, 7, 15), ZoneId.of("Asia/Colombo")) returns 24. As expected
  • getHoursOfDay(LocalDate.of(2014, 7, 15), ZoneId.of("Europe/Oslo")) also returns 24.
  • But here comes a funny version: getHoursOfDay(LocalDate.of(2014, 3, 30), ZoneId.of("Europe/Oslo")) returns 23! This is daylight saving time.
  • Similarly: getHoursOfDay(LocalDate.of(2014, 10, 26), ZoneId.of("Europe/Oslo")) also returns 25
  • And of course, down under, everything is upside down: getHoursOfDay(LocalDate.of(2014, 10, 5), ZoneId.of("Australia/Melbourne")) gives 23.
  • Except, of course, in Queensland: getHoursOfDay(LocalDate.of(2014, 10, 5), ZoneId.of("Australia/Queensland")) => 24.

Daylight saving hours: The bane of programmers!

Daylight saving hours were instituted with the stated purpose of improving worker productivity by providing more working hours with light. Numerous studies have failed to prove that it works as intended.

Instead, when I examined the history of daylight saving hours in Norway, it turns out that it was lobbied by a golfer and a butterfly collector (“lepidopterist”) so that they could better pursue their hobbies after working hours. Thus the name of this blog post.

Most of the time, you can ignore daylight saving hours. But when you can’t, it can really bite you in the behind. For example: What does the hour by hour production of a power plan look like on the day that changes from daylight saving hours to standard time? Another example given to me by a colleague: TV schedules. It turns out that some TV channels just can’t be bothered to show programming during the extra hour in the fall. Or they will show the same hour of programming twice.

The Joda-Time API and now, the Java 8 time API java.time can help. If you use it correctly. Here is the code to display a table of values per hour:

void displayHourlyTable(LocalDate date, ZoneId zoneId) {
    ZonedDateTime startOfDay = date.atStartOfDay(zoneId);
    ZonedDateTime end = startOfDay.plusDays(1);
    for (ZonedDateTime current = startOfDay; current.isBefore(end); current = current.plusHours(1)) {
        System.out.println(current.toLocalTime() + ": " + current.toInstant());

Given 2014/10/26 and Oslo, this prints:

00:00: 2014-10-25T22:00:00Z
01:00: 2014-10-25T23:00:00Z
02:00: 2014-10-26T00:00:00Z
02:00: 2014-10-26T01:00:00Z
03:00: 2014-10-26T02:00:00Z

And on 2014/3/30, it prints:

00:00: 2014-03-29T23:00:00Z
01:00: 2014-03-30T00:00:00Z
03:00: 2014-03-30T01:00:00Z
04:00: 2014-03-30T02:00:00Z

So, if you ever find yourself writing code like this: for (int hour=0; hour<24; hour++) doSomething(midnight.plusHours(hour)); you may want to reconsider! This code will (probably) break twice a year.

At the face of it, time is an easy concept. When you start looking into the details, there’s a reason that the java.time library contains 20 classes (if you don’t count the subpackages). When used correctly, time calculations are simple. When used incorrectly, time calculations look simple, but contain subtle bugs.

Next time, perhaps I should ruminate on the finer points of Week Numbers.

Johannes Brodwall

Johannes works as a programmer, software architect and provocateur for Sopra Steria Norway. He loves writing code in Java, C# and JavaScript and making people think.
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