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.

That syntax has worked since at least MS-DOS 2.1 (the earliest version I still have a virtual machine for).

I dont know where the two-comma version came from, but it most likely exploited a parsing glitch in COMMAND.COM, and somehow this variant gained traction and became the version everybody used (even though the other version is two keystrokes shorter). As a result, this weird syntax has become grandfathered as a special-case in the CMD.EXE parser. Here's some actual code from the part of CMD.EXE which parses the arguments to the COPY command:

if (parse_state == SEEN_TWO_COMMAS)
    copy_mode = TOUCH;