Correct syntax for toToday()?

Try this:

val today_start = (entrydoormonitor_start.state as DateTimeType).zonedDateTime.with(LocalDate.now)
entrydoormonitor_start.postUpdate(today_start.toString)

FYI in JRuby, working with date/time/duration is more intuitive yet simplified, so that a lot of type castings are not needed.

today_start = entrydoormonitor_start.state.with(LocalDate.now)
entrydoormonitor_start.update(today_start)

You could even write a method to bring any datetime item to today

def to_today(item) = item.update(item.state.with(LocalDate.now))

to_today(entrydoormonitor_start)
to_today(entrydoormonitor_end)

Or even like this

class DateTimeItem
  def to_today = update(state.with(LocalDate.now))
end

entrydoormonitor_start.to_today
entrydoormonitor_end.to_today