I set up a publishing project including a custom postamble :html-postamble "Date: %d, Last modification: %C". When I export this file The %d date is empty. From `C-h v org-html-postamble-format` I got the info about the available elements to use:

```
%t stands for the title.
%s stands for the subtitle.
%a stands for the author's name.
%e stands for the author's email.
%d stands for the date.
%c will be replaced by `org-html-creator-string'.
%v will be replaced by `org-html-validation-link'.
%T will be replaced by the export time.
%C will be replaced by the last modification time.
```

I tried to set both, :html-postamble and :html-postamble-format. Which date is meant with the %d variable? Do I've to set it manually? I thought %d will output the creation date of the file itself and %C the last updated time (btw I get german weekdays and idk why & how to change).