//Downloading data on a background thread above this line
DispatchQueue.main.async {
   //Go through all the recently downloaded customers
   for (index, customer) in customers.enumerated() {
      //for each, add a new customer to core data
      self.createNewCustomer(customer: customer)
   }
}

func createNewCustomer(customer: Customer) {
   let newCustomer = Customers(context: self.context)
   ...
   //Convert time to seconds using a method in a different view controller.
   let seconds = CustomerViewController().secondsFromDate(date: date)
   //Save to core data and repeat
   appDelegate.saveContext()
}