OK, let’s try to get to the root of the problem. These are two example’s … 1st doesn’t work, 2nd works! The question is why?
A_datatime_item.postUpdate((A_datatime_item.state as DateTimeType).plusHours(2))
A_datatime_item.postUpdate(new DateTime((A_datatime_item.state as DateTimeType).zonedDateTime.toInstant().toEpochMilli).plusHours(2).toString)
The 1st example is taken from this thread. So it worked in Jan 2016.