@Override
    protected StateFactory init(ExpressionExecutor[] attributeExpressionExecutors, ConfigReader configReader,
                                SiddhiQueryContext siddhiQueryContext) {
        if (attributeExpressionExecutors.length != 1) {
            throw new SiddhiAppValidationException("incrementalAggregator:timestampInMilliseconds() function " +
                    "accepts only one argument, but found " + attributeExpressionExecutors.length);
        }
        if (attributeExpressionExecutors[0].getReturnType() != Attribute.Type.STRING) {
            throw new SiddhiAppValidationException("Only string values can be converted to unix time, but found " +
                    attributeExpressionExecutors[0].getReturnType());
        }
        return null;
    }
Executor class for converting string timestamp to unix time in incremental processing.
Condition evaluation logic is implemented within executor.
modules/siddhi-core/src/main/java/io/siddhi/core/executor/incremental/IncrementalUnixTimeFunctionExecutor.java