fix: MongoDB aggregation pipeline with $dateSubtract from $$NOW returns no results

spec/ParseQuery.Aggregate.spec.js

@@ -439,6 +439,36 @@ describe('Parse.Query Aggregate testing', () => {
     }
   );
 
+  it_id('3723671d-4100-4103-ad9c-60e4c22e20ff')(it_exclude_dbs(['postgres']))('matches expression with $dateSubtract from $$NOW', async () => {
+    const obj1 = new TestObject({ date: new Date(new Date().getTime() - 1 * 24 * 60 * 60 * 1_000) }); // 1 day ago
+    const obj2 = new TestObject({ date: new Date(new Date().getTime() - 2 * 24 * 60 * 60 * 1_000) }); // 3 days ago
+    await Parse.Object.saveAll([obj1, obj2]);
+
+    const pipeline = [
+      {
+        $match: {
+          $expr: {
+            $gte: [
+              '$date',
+              {
+                $dateSubtract: {
+                  startDate: '$$NOW',
+                  unit: 'day',
+                  amount: 2,
+                },
+              },
+            ],
+          },
+        },
+      },
+    ];
+
+    const query = new Parse.Query('TestObject');
+    const results = await query.aggregate(pipeline, { useMasterKey: true });
+    expect(results.length).toBe(1);
+    expect(new Date(results[0].date.iso)).toEqual(obj1.get('date'));
+  });
+
   it_only_db('postgres')(
     'can group by any date field (it does not work if you have dirty data)', // rows in your collection with non date data in the field that is supposed to be a date
     done => {

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -960,23 +960,28 @@ export class MongoStorageAdapter implements StorageAdapter {
     return pipeline;
   }
 
-  // This function will attempt to convert the provided value to a Date object. Since this is part
-  // of an aggregation pipeline, the value can either be a string or it can be another object with
-  // an operator in it (like $gt, $lt, etc). Because of this I felt it was easier to make this a
-  // recursive method to traverse down to the "leaf node" which is going to be the string.
+  /**
+   * Recursively converts values to Date objects. Since the passed object is part of an aggregation
+   * pipeline and can contain various logic operators (like $gt, $lt, etc), this function will
+   * traverse the object and convert any strings that can be parsed as dates into Date objects.
+   * @param {any} value The value to convert.
+   * @returns {any} The original value if not convertible to Date, or a Date object if it is.
+   */
   _convertToDate(value: any): any {
     if (value instanceof Date) {
       return value;
     }
     if (typeof value === 'string') {
-      return new Date(value);
+      return isNaN(Date.parse(value)) ? value : new Date(value);
     }
-
-    const returnValue = {};
-    for (const field in value) {
-      returnValue[field] = this._convertToDate(value[field]);
+    if (typeof value === 'object') {
+      const returnValue = {};
+      for (const field in value) {
+        returnValue[field] = this._convertToDate(value[field]);
+      }
+      return returnValue;
     }
-    return returnValue;
+    return value;
   }
 
   _parseReadPreference(readPreference: ?string): ?string {