@Override
    protected void doExecute(ExecutionContext context) {
        String inputString = String.valueOf(context.getCurrentValue());
        long epochTime =  Instant.parse(inputString).getEpochSecond();
        context.setCurrentValue(new LongFieldValue(epochTime));
    }
Converts ISO-8601 formatted date string to UNIX Epoch Time in seconds

@author bergum
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/ToEpochSecondExpression.java