Issue 1104: Explicitly limit the printed string to 12 characters

Issue 1104: Explicitly limit the printed string to 12 characters

GCC8 tries to diagnose `snprintf()` overflows but isn't quite
smart enough for this case, so emits a false-positive warning.

Remember that `%12s` only specifies the minimum number of bytes.  GCC8
conservatively assumes this might result in writing the full length of
`date2`.  (Which will never be longer than 12 bytes, but GCC8
apparently can't reason about `strftime` format specifiers yet.)
Changing the specifier here to `%12.12s` explicitly truncates to 12
bytes and should help the compiler understand that this will never
overflow.

While I'm here, correct a minor typo in the previous line; it used
`sizeof(date)` instead of `sizeof(date2)`.  (Both are the same
size, so this had no functional impact.)