In my last post I tried to explain the basics of Daylight Saving Time (DST) and the effect it has on conversions from GMT to local time. Now I want to go into more detail about local time and some issues that DST causes.

Because of DST, local time is not continuous in the spring and repeats in the fall. For example, what if you have a task you want to schedule your computer to run at 02:15 in the morning. What happens if you try to pick the morning that DST goes into effect? The computers clock will go from 01:59 to 03:00 without ever hitting 02:15. This means that the computer should not allow you to pick any time between 02:00 and 02:59 on that day.

The situation in the Fall when DST ends is even more confusing. This time the computers clock will go from 01:59 to 02:00 just fine, but when it changes from 02:59 the first time it will switch to 02:00 again. That’s right, when expressed in local time, the hour from 02:00 to 02:59 exists twice on the day that DST ends. As a computer programmer, what would you do? I guess you could ask the user: “Which 02:15 do you want, the first one or the second one?”

[tags]DST, Daylight Saving Time, GMT, Greenwich Mean Time, UTC, Coordinated Universal Time, time zone[/tags]