The magic incantation for updating the last-modified date on a file is
COPY /B FILE+,,
What strange syntax! What's with the plus sign and the commas, anyway?
The formal syntax is the much more straightforward
COPY /B A+B+C+D
This means to start with the file A, then append the files B, C, and D, treating them all as binary files.
If you omit the B+C+D part, then you get
COPY /B A+
This means "Start with A, then append nothing." The side effect is that the last-write time gets updated, because the command processor opens A for append, writes nothing, then closes the handle.
Read more: The old new thing
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