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Daylight Savings Time and Macintosh Extended Format

As portions of the United States sprung ahead to daylight savings time last Saturday, reports of problems on Macintosh Extended Format (HFS Plus) drives began to surface. (Extended Format is an optional aspect of Mac OS 8.1; for more details, see "All About Macintosh Extended Format (HFS Plus)" in TidBITS 414.) As noted in an Apple Tech Note, disks formatted in Extended Format store file creation and modification times as an offset from Greenwich Mean Time. When you check or uncheck the Daylight Savings Time checkbox in the Date & Time control panel, the offset changes correspondingly. On an Extended Format disk, this changes the creation and modification times for files; dates may change as well. Some people won’t notice the change, but others may have problems; in particular, backups may take longer or fill backup media. A possible solution is to change the time using the Current Time field in the Date & Time control panel instead of the Daylight Savings Time checkbox. Another fix comes in the form of HFS+ DST Timefix, a tiny extension from Glenn Austin that causes Extended Format to ignore the fact that the Daylight Savings Time checkbox is checked.

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