How to add number of days to particular Date in Swift

Question

I am getting date from JSON and i need to add some days to that date

"flash_packages": [
        {
            "id": 78,
            "duration": 20,
            "purchase_date": "2022-05-26",

here i need to add duration to purchase_date here duration is number of days

code: here i have turned json date string to date using dateOnly()..here how to add 20 days to this date 26.05.2022

     func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

      let cell = tableView.dequeueReusableCell(withIdentifier: "FlashTableViewCell", for: indexPath) as! FlashTableViewCell

         let indexData = viewBooster?.result?.flash_packages?[indexPath.row]
        
        let purchasedDate = indexData?.purchase_date?.dateOnly() ?? ""
        let duration = indexData?.duration ?? 0

        print("Purchased date:  \(purchasedDate), and the duration is \(duration) days")

        return cell
     }

here how to add 20 days to this date 26.05.2022, please guide me.

0/p:

Purchased date: 26.05.2022, and the duration is: 20 days