What is the precision of *.datenum, which dir() returns

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Mads
Mads 2018-5-7
评论： Jan 2018-5-8
When I apply dir() to a file I get a struct with folder, name, date, bytes, and also datenum. How many significant digits do I need to fully store the number correctly, e.g., when I need to compare two files...
Say I print the number to an ascii file: fprintf(fid,'%.xf',mystruct.datenum) what is x supposed to be?

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回答（2 个）

dpb
dpb 2018-5-7
datenum is a double; hence has 15-16 significant digits; the safest way to store it is as a double but you'll get as close as can with '%.16g'
Jan
Jan 2018-5-7
编辑：Jan 2018-5-7
The precision of the date depends on the file system. Linux' ext4 file system supports nanoseconds, NTFS uses 100 nanoseconds, FAT has a resolution of 2 seconds.
Under Windows FEX: GetFileTime can reply the native UINT64 file times:
T = GetFileTime(FileName, 'native')
T.Write
>> 131499596832223845
Using this integer avoids rounding and precision problems of the doubles used for Matlab's datenum format.
  2 个评论
Mads
Mads 2018-5-8
Interesting discussion. What if a jpeg was captured on a Nikon or smartphone, and stored on a Linux system or mac? Do datenums get rounded when transfered?
Is there a getfiletime for matlab running on mac?
Jan
Jan 2018-5-8
There are two possibilities: Either moving the file keeps the dates, that the time is rounded to the precision of the file system. Or the function to move the file (e.g. the operating system) uses the current date as date of creation.

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