"zhilizhao(赵志立)" (12021-12-29):
> How about add a restriction like this:
> 
> if (format.endsWith(“%S"))
>     enable the feature
> else
>     warning message
> 
> It’s a useful feature, it shouldn't create unexpected results, but
> doesn’t need to support every use case.

I would not oppose it, but I find it inelegant, especially because it
requires a different expansion function, localtime_ms instead of
localtime.

What about this: with the original function "localtime", if the format
ends in "%3N", then append the millisecond. It can later be expanded to
support %xN at any place in the format for any value of x.

Regards,

-- 
  Nicolas George