private static ExpressionExecutor getTimeStampExecutor(SiddhiQueryContext siddhiQueryContext,
                                                           Map<String, Table> tableMap,
                                                           List<VariableExpressionExecutor> variableExpressionExecutors,
                                                           MetaStreamEvent metaStreamEvent) {

        Expression timestampExpression;
        ExpressionExecutor timestampExecutor;

        // Execution is based on system time, the GMT time zone would be used.
        timestampExpression = AttributeFunction.function("currentTimeMillis", null);
        timestampExecutor = ExpressionParser.parseExpression(timestampExpression, metaStreamEvent, 0, tableMap,
                variableExpressionExecutors, false, 0, ProcessingMode.BATCH, false, siddhiQueryContext);
        return timestampExecutor;
    }
This is the parser class of incremental aggregation definition.
modules/siddhi-core/src/main/java/io/siddhi/core/util/parser/AggregationParser.java