Spring Data MongoDB month aggregation

.java

Criteria criteria = new Criteria().andOperator(
	where("warehouse").is(warehouse),
	where("date").gte(currentYear.getStart()),
	where("date").lte(currentYear.getEnd())
);
 
ProjectionOperation dateProjection = project()
	.and("amount").as("amount")
	.and("date").extractYear().as("year")
	.and("date").extractMonth().as("month");
 
GroupOperation groupBy = group("year", "month")
	.sum("amount").as("total")
	.count().as("count");
 
Aggregation agg = newAggregation(
	match(criteria),
	dateProjection,
	groupBy,
	sort(Sort.Direction.ASC, "year", "month")
);
 
//Convert the aggregation result into a List
AggregationResults<InvoiceHistory> groupResults = mongoTemplate.aggregate(agg, Invoice.class, InvoiceHistory.class);

and what is the type of date?