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Daylight Savings Time settings in Mac OS X

Notes on how to check the Daylight Savings Time settings on your Mac.
Last updated: 2 October 2009

The conditions for the beginning and end of Daylight Savings Time (DST) in the United States have changed, starting in 2007. One way you can check whether or not your system will make the change correctly is to open Terminal.app and give the following command:

 zdump -v /etc/localtime | grep 2007
This should produce something like:
/etc/localtime  Sun Mar 11 06:59:59 2007 UTC = Sun Mar 11 01:59:59 2007 EST isdst=0 gmtoff=-18000
/etc/localtime  Sun Mar 11 07:00:00 2007 UTC = Sun Mar 11 03:00:00 2007 EDT isdst=1 gmtoff=-14400
/etc/localtime  Sun Nov  4 05:59:59 2007 UTC = Sun Nov  4 01:59:59 2007 EDT isdst=1 gmtoff=-14400
/etc/localtime  Sun Nov  4 06:00:00 2007 UTC = Sun Nov  4 01:00:00 2007 EST isdst=0 gmtoff=-18000
If you don't see DST starting on March 11 2007 then you need to update your settings. There is a Software Update available from Apple which will do this for you, so the setting should be correct if you run Software Update regularly.

(Note that the possibility of using a full-path filename on the zdump command line in place of a timezone name is not documented in the zdump man page.)
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