Date arithmetics sometimes works incorrectly in timezones with daylight saving time.

[piotrszul]

If the JVM timezone is set to 'Europe/Warsaw'

'2017-01-01T00:00:00Z' + '9 months' results in '2017-09-30T23:00:00Z' not as expected '2017-10-01T00:00:00Z'

This is cause by a bug in DateTimeType .add() operation (reported here: hapifhir/hapi-fhir#3979)