Thank you, additionally I found out, that these are strftime formatting variables: https://apidock.com/ruby/DateTime/strftime
My example use case (Day plus ISO8601 date plus time with seconds plus calendar week number):
%A, %F %T, KW%V
Tuesday, 2017-07-18 13:58:51, KW29
Just trying if some fancy formatting may be useful for daily work:
<small>%A, %Y-</small>%m-<b>%d</b> %T, <small>KW%V</small>
Tuesday, 2017-07-18 13:58:51, KW29