Currently there is no way to convert types without the aggregation framework.

This would be useful to allow things like Dates <-> Timestamps/Strings, Strings <-> Numbers and even cooler stuff like OID <-> String.

I think the most intuitive way to add type casting would be to reuse the $type query in the $project stage of the aggregation pipeline which would maybe look something like this:

db.col.aggregate([{
	$project: {
		my_date: { $type: ["$my_date", 16] }
	}
}])

Where "$my_date" is currently a `Date` bson type and will be casted to a 32-bit integer (16).

Perhaps aliases could also be supported to allow for:

db.col.aggregate([{
	$project: {
		my_date: { $type: ["$my_date","int"] }
	}
}])

Ultimately syntax doesn't matter but I think type casting is something the aggregation framework is lacking which would help many use cases without relying on in app implementations.