MongoDB - Sum aggregation with conditionals

Question

I am working on aggregation to perform some analytics on an Orders object, I want to get the total price from every order from the previous month, and the total from this month, I tried passing in the $match parameter, with the appropriate time conditionals but it's not aggregating any of the data. even though if I perform a countDocuments query with the same time conditionals I get a number of documents returned.

const totalThis = await Order.aggregate([
  {
    $match: {
      // should cut the amount of orders off at the beginning of the current month
      createdAt: { $gte: thisMonth.getMonth() + 1 },
    },
  },
  {
    $group: {
      // using $group, _id has to be there or it doesnt
      // return anything, though we dont need an _id
      _id: null,
      total: {
        $sum: "$totalPrice",
      },
    },
  },
]);
res.status(200).json({
  lastMonth: lastMonthSales,
  thisMonth: thisMonthSales,
  totalSalesLast: totalLast,
  totalSalesThis: totalThis,
});

In my Postman request, lastMonth returns 0, so it would make sense that the array returned to totalSalesLast would be empty, however thisMonth returns 5 documents, so it should work. Obviously, if I remove the $match conditional it will aggregate all the documents, but that's because it's doing ALL the documents and not actually filtering.