Thank you.
I was searching this topic for a long time. As I hoped for a solution in Rules DSL I did not dig down into the rules-tools.
The rules tools are very interesting - I think I will have to learn JavaScrips soon 
For anybody who currently sticks to RulesDSL, I found the solution of my problem here combined with here
val MyJavaTimeFromDateTimeItem = (MyDateTimeItem.state as DateTimeType).getZonedDateTime()
var TimeOfToday = MyJavaTimeFromDateTimeItem.withYear(now.getYear()).withMonth(now.getMonthValue()).withDayOfMonth(now.getDayOfMonth())
logInfo("Test", "The new time is: " + TimeOfToday )
BR Peter