Over the last week I have had to explain to many people why this years Daylight Saving Time (DST) changes are such a big deal. While the result is simply moving the clock ahead one hour in the spring and moving it back an hour in the fall, the behind the scenes details are more complex.

Lets start by talking about what DST means to a computer. While all modern operating systems think in UTC/GMT those of us using them think in local time. To get from GMT to local time you simply take GMT and add the local time bias. Here in Missouri, the local time bias is -6 (Central Standard Time or GMT-6) during the winter months and -5 the rest of the year when DST is in effect (Central Daylight Time or GMT-6+1).

The first real complication comes in knowing when DST in in effect. Prior to this year’s change, DST went into effect at 02:00 local time on the first Saturday in April and ended at 02:00 on the last Sunday of October. This year the start and end of DST has been changed to 02:00 on the second Sunday in March and 02:00 on the first Sunday in November. Since every computer has to know when to change the local time bias, every computer had to be updated with the new information.

Next time I will cover the complications caused by this process when you try to use local time to calculating an arbitrary point of time in the future.

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