Use Setpoint for changing times

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Dear all

I’m using a KNX weather station for many different things, also as a time controller since it provides many functionalities out of the box. It provides time slots to steer different actions. Every time slot consists of a start time and an end time. Default values are set, e.g. start time = 20:15:00 and end time = 06:45:00.

Now I want to change that times via my sitemap with setpoint items. For that reason I have read this Link and tried to adjust it to my needs but I’m still struggeling to get it running.

My items:

DateTime	Wochenschaltuhr_Zeitraum_24_ON	"Wochenschaltuhr Zeitraum 24: ON [%1$tT]"							<wetterstation>		(gEMPTY)	{knx="<10.001:7/0/92"}	//KO 1303	//[10.001] 3 Byte
DateTime	Wochenschaltuhr_Zeitraum_24_OFF	"Wochenschaltuhr Zeitraum 24: OFF [%1$tT]"							<wetterstation>		(gEMPTY)	{knx="<10.001:7/0/93"}	//KO 1304	//[10.001] 3 Byte
Number     TimeSlot24_Minutes  				"Verändere Zeit in Minuten"     
String     TimeSlot24_Time      			"Zeitslot 24 Zeitanzeige [%s]"

My rule

import org.openhab.core.library.types.*
import org.openhab.core.persistence.*
import org.openhab.model.script.actions.*
import org.joda.time.*

rule "Setpoint to change time for TimeSlot24"
when
    Item TimeSlot24_Minutes received update
then
    if(TimeSlot24_Minutes.state instanceof DecimalType) {
    	var int minutes = (TimeSlot24_Minutes.state as DecimalType).intValue
    	}

    var DateTime Wochenschaltuhr_Zeitraum_24_ON = parse(now.getYear() + "-" + now.getMonthOfYear() + "-" + now.getDayOfMonth() + "T00:00")

    // add the number of minutes selected
    Wochenschaltuhr_Zeitraum_24_ON = Wochenschaltuhr_Zeitraum_24_ON.plusMinutes(minutes)

    // update the alarm display time    
    Wochenschaltuhr_Zeitraum_24_ON.sendCommand(String::format("%02d:%02d", Wochenschaltuhr_Zeitraum_24_ON.getHourOfDay(), Wochenschaltuhr_Zeitraum_24_ON.getMinuteOfHour()))
end

My sitemap

Text item=Wochenschaltuhr_Zeitraum_24_ON
Setpoint item=TimeSlot24_Minutes minValue=0 maxValue=600 step=5

In my logfile I can see the following error when clicking on the setpoint button:

2016-12-25 18:23:19.345 [ERROR] [o.o.c.s.ScriptExecutionThread ] - Error during the execution of rule 'Setpoint to change time for TimeSlot24': The name '<XFeatureCallImplCustom>.plusMinutes(<XFeatureCallImplCustom>)' cannot be resolved to an item or type.

I don’t understand this error. What is wrong in my rule?

Another thing (assumption?):
When trying to change the item Wochenschaltuhr_Zeitraum_24_OFF via the KNX software (ETS4) I can see that the changed value looks like that:
2016-12-25 19:13:38 - Wochenschaltuhr_Zeitraum_24_OFF received command 1970-01-04T06:45:00

I assume now that my rule does not provide the correct format that has to be handed over to the binding.

Any ideas how I can get it running?

Thanks & regards
John