We can use $subtract operator to subtract tow numbers. $subtract is arithmetic aggregation operator which subtract two number and returns the difference.
$subtract can be used to get differences between dates means we can subtract date also, $subtract treat to the second number as millisecond if the first number is date type.
Syntax:
{ $subtract: [ <expression1>, <expression2> ] }

Example: Suppose we have a collection “Inventory” as below and before showing the details of the product on our website we need to calculate the total price in our query.
{
_id:1,
Title:"Title1",
SKU:"1234567890123",
Price:20.50,
ShippingCharge:4.80,
Discount:2.00,
CreatedDate:ISODate("20160809T013:00:00Z")
}
{
_id:2,
Title:"Title2",
SKU:"1234567890121",
Price:30.50,
ShippingCharge:4.80,
Discount:2.00,
CreatedDate:ISODate("20160809T013:00:00Z")
}
{
_id:3,
Title:"Title3",
SKU:"1234567890122",
Price:50.50,
ShippingCharge:4.80,
Discount:2.00,
CreatedDate:ISODate("20160809T013:00:00Z")
}

See the following query here we calculating the discounted price. We are adding Price and ShippingCharge and subtract discounted price.
db.Inventory.aggregate(
[
{ $project:
{ Title: 1,SKU:1, Discounted_Price:
{ $subtract:
[ { $add: [ "$Price", "$ShippingCharge" ] }, "$Discount" ]
}
}
}
] )

Result:
/* 1 */
{
"_id" : 1,
"Title" : "Title1",
"SKU" : "1234567890123",
"Discounted_Price" : 23.3
}
/* 2 */
{
"_id" : 2,
"Title" : "Title2",
"SKU" : "1234567890121",
"Discounted_Price" : 33.3
}
/* 3 */
{
"_id" : 3,
"Title" : "Title3",
"SKU" : "1234567890122",
"Discounted_Price" : 53.3
}

Same like that if you want to subtract date you can use $subtract operator.
Subtract one day from CreatedDate
See the following query which is subtracted one day from CreatedDate.
db.Inventory.aggregate(
[
{ $project:
{ Title: 1,SKU:1, Sub_Date:
{ $subtract:
[ "$CreatedDate", 24*60*60*1000 ]
}
}
}
] )

Result:
/* 1 */
{
"_id" : 1,
"Title" : "Title1",
"SKU" : "1234567890123",
"Sub_Date" : ISODate("20160808T01:00:00.000Z")
}
/* 2 */
{
"_id" : 2,
"Title" : "Title2",
"SKU" : "1234567890121",
"Sub_Date" : ISODate("20160808T01:00:00.000Z")
}
/* 3 */
{
"_id" : 3,
"Title" : "Title3",
"SKU" : "1234567890122",
"Sub_Date" : ISODate("20160808T01:00:00.000Z")
}
