how to add 7 days in date manually using mongodb(aggregation)?

I have a query in which I'm getting data using start Date and end Date. And it's working perfectly fine. now I want to add 7 days in my start date using aggregation and want to get data of start Date + 7 added days. i have tried the below code but couldn't get any success

  {
    $match: {
      publishedAt: {
        $lt: new Date(startDate),
        $gte: new Date(endDate),
      },
    },
  },
  {
    $set: {
      publishedAt: {
        $add: [new Date(startDate), 1000 * 60 * 60 * 24],
      },
    },
  },

Use this:

   {
      $set: {
         publishedAt: {
            $dateAdd: {
               startDate: new Date(startDate),
               unit: "day",
               amount: 7
            }
         }
      }
   }

Or use a 3rd party library, e.g. Luxon

   {
      $set: {
         publishedAt: DateTime.now().plus({ days: 7 }).toJSDate()
      }
   }