Currently, method such as File.GetLastWriteTime and FileInfo.LastWriteTime return timestamps which are only accurate to the nearest second on Unix. On OSX, this seems to be a system-level limitation in HFS+. On most Linux filesystems, however, millisecond-level information does seem to be available, at least via stat. @stephentoub recommended that we could use utimes rather than utime in the native shim in order to retrieve this information.

This also seems to affect files copied by File.Copy, because we copy over some timestamp metadata from the source file (to match Windows).