Every time I save a file, Vim warns about the file being changed
outside of Vim. Why? If you get the following warning message, every time you save a file:
WARNING: The file has been changed since reading it!!! Do you really want to write to it (y/n)?
then this problem could be related to a bug in MS-Windows on the day daylight saving time starts. Vim remembers the timestamp of the file after it was written. Just before the next write the timestamp is obtained again to check if the file was changed outside of Vim. This works correctly, except on the day daylight saving time starts. This problem will go away the next day after the day the daylight saving time starts.