Hello watou,
great, that worked. There were some other errors in the code, so pasting working solution for completeness:
import org.joda.time.*
...
var String tst = transform("JSONPATH", "$.tst", json)
var DateTime time = new DateTime(Long::parseLong(tst) * 1000L)
lastStatusUpdate.postUpdate(new DateTimeType(time.toString))
Thank you for your prompt help
Peter