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

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Mads
Mads on 7 May 2018
Commented: Jan on 8 May 2018
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?

Answers (2)

dpb
dpb on 7 May 2018
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'
  1 Comment
Walter Roberson
Walter Roberson on 7 May 2018
Since it is a double, use num2hex() to convert it to hex.

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Jan
Jan on 7 May 2018
Edited: Jan on 7 May 2018
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 Comments
Jan
Jan on 8 May 2018
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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