@Override
    public DataType setInputType(DataType input, TypeContext context) {
        super.setInputType(input, DataType.STRING, context);
        return DataType.LONG;
    }
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