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A general description is here.
‘Sys.setFileTime()’ will set fractions of seconds on platforms which
support it (not currently Windows nor macOS).
‘Sys.setFileTime()’ and ‘file.copy(copy.date = TRUE)’ will set timestamps with fractions of seconds on platforms/filesystems which support this.
(Windows only.) ‘file.info()’ now returns file timestamps including fractions of seconds; it has done so on other systems since R 2.14.0.
If available, the POSIX 2008 system call ‘utimensat’ will be used by ‘setFileTime()’ and ‘file.copy()’. This may result in slightly more accurate file times.
‘c()’'s argument ‘use.names’ is documented now, as belonging to the (C internal) default method. In “parallel”, argument ‘recursive’ is also moved from the generic to the default method, such that the formal argument list of ‘base’ generic ‘c()’ is just ‘(...)’.