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Have a few thousand tiff photos I’ve scanned in over the past few years into Lightroom Classic. For reasons I won’t go into here since not relevant to the question, I’d like to change the year for all photos to be 45 years earlier than what they currently are but want the month day hour and seconds to all stay as they are.

Lightroom has an ExifTool plugin but the docs don’t have an example for just shifting only the year back by a certain number.

Something maybe like, exiftool "-AllDates-=45:0:0:0:0”, but with only changing year.

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You're very close. By default, time shifts assume the shift is in hours unless there's a space between numbers. Try this:
exiftool "-AllDates-=45:0:0 0" /path/to/files/

This will shift the three most common EXIF tags, DateTimeOriginal, CreateDate, and ModifyDate. It will not affect any other time based tags. You would have to specify them separately.

Additionally, on Windows, there are problems if you try to set the file system timestamps, FileCreateDate and FileModifyDate, back before circa 1970. This is due to some problems with the underlying Perl routines that exiftool is based upon.

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  • For many dates in Windows NT, the world did not exist until the "zero" date in NT domains which was January 1, 1970. Any date that was undefined, or had the default zero value, was interpreted as 12:00 AM January 1, 1970. The "zero" date for many current Windows implementations is 12:00 AM December 30, 1899. – Michael C Nov 26 '20 at 6:47
  • The thing about the Perl routines that are the problem is that the range is weird. You can set a Windows file system time stamp with exiftool to 1899:12:31 23:59:59 but anything from 1900:01:01 00:00:00 to 1970:12:31 23:59:59 will be inaccurate. And the really weird thing is that the interval is increasing. Three years ago 1967 was the year that was corrupted and 1968 worked. – StarGeek Nov 26 '20 at 7:16
  • That's fascinating! – Michael C Nov 26 '20 at 7:25
  • Thank you! I’ve only used Mac’s for the past 15 years or so, so as long as the early dates are compatible with some non Mac destinations including Lightroom website, Google photos. – cchiera Nov 26 '20 at 16:32

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