From 12.9 and above, and if you apply fix T51S038 to 12.8CF3, you need to replace the following:
_eventContextInformation.getSecondaryObjectAttribute("START_DATE", "");
_eventContextInformation.getSecondaryObjectAttribute("VALID_UNTIL", "");
With:
_eventContextInformation.getSecondaryObjectAttributeForDate("START_DATE", "");
_eventContextInformation.getSecondaryObjectAttributeForDate("VALID_UNTIL", "");
This will insure correct conversion of datetimes to localtime.