97 Things Every Java Programmer Should Know

Chapter 18. Do You Know What Time It Is?

Christin Gorman

At what time does the Scandinavian Airlines plane from Oslo to Athens arrive on Monday? Why are questions that seem so easy in day-to-day life so difficult in programming? Time should be simple, just seconds passing, something a computer is very good at measuring:

System.currentTimeMillis() = 1570964561568

Although correct, 1570964561568 is not what we want when we ask what time it is. We prefer 1:15 p.m., October 13, 2019.

It turns out that time is two separate things. On the one hand, we have seconds passing. On the other, we have an unhappy marriage between astronomy and politics. Answering the question “What time is it?” depends on the location of the sun in the sky relative to your position along with the political decisions made in that region up to that point in time.

Many of the problems we have with date and time in code come from mixing these two concepts. Using the latest java.time library (or Noda Time in .NET) will help you. Here are three main concepts to help you reason correctly about time: LocalDateTime, ZonedDateTime, and Instant.

LocalDateTime refers to the concept 1:15 p.m., October 13, 2019. There can be any number of these on the timeline. Instant refers to a specific point on the timeline. It is the same in Boston as in Beijing. To get from a LocalDateTime to  an Instant, we need a TimeZone, which ...