That was it, thanks @spy0r
"now"is Java time type so the correct conversion should be as described here :
val userStartTime = parse(now.getYear() + "-" + now.getMonthValue() + "-" + now.getDayOfMonth() + "T" + itmIrrigationStartTime.state + ":00.000+01:00").withZoneSameInstant(ZoneId.systemDefault())