Daylight Savings Time and Unix Cron

Until recently I was of the opinion that, in Ireland, when the clocks change in Spring and Autumn to account for Daylight Savings Time, they always do so at 2am. Having never given it too much thought over the years, I was surprised to find out last weekend that the time they actually change differs depending on whether the clocks are going forward (Spring) or backward (Autumn).

In Ireland, the clocks go forward one hour at 1am (in Spring) but when they go back again in Autumn, they actually do so at 2am. You can confirm this for yourself on the Greenwich Meantime site for Ireland.

There is an important lesson in this too for administrators of the Unix operating system. Never schedule an import cron task between the hours of 1:00am and 2:00am because if you do, it will not run on the weekend when DST comes into effect and it may be run twice on the weekend where DST ends.

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