Timer to count time since a timestamp - ios

Question

I have a table view containing a dynamic number of UITableViewCells. The data that populates the cell originates from Firebase. Each record (and therefore each cell) has a field for recordTimestamp which is the Unix timestamp for when that record was added to Firebase.
I'm trying to find a way/best practice for making a "counter" in each cell that updates every second to show how many minutes/seconds it has been since recordTimestamp. The counter should continue to increment every second until a button in the cell is tapped.
I've tried using a timer object to call a function once per second which compares recordTimestamp against the current time. Then, on button tap it'd fire timer.invalidate. This method kind of worked, but after about 20 seconds of running the counters were getting all out of sync and some were a few seconds behind. It was also incredibly laggy - there are only about 10 rows in the table and you could barely scroll smoothly.
Any suggestions for achieving this?
EDIT: Code as requested. For now, I'm just trying to get it to count from 0 properly. Once I get that sorted I'll add the logic to first calculate the time difference and then increment every second.
class newRequestCell: UITableViewCell {
#IBOutlet weak var acceptButton: UIButton!
#IBOutlet weak var timeSinceRequest: UILabel!
var timer = Timer()
var counter = 0
func setRequestCell(customerRequest: customerRequest) {
customerName.text = String(customerRequest.customerName)
orderNumber.text = String(customerRequest.orderNumber)
timer = Timer.scheduledTimer(timeInterval: 1, target:self, selector: #selector(self.updateCounter), userInfo: nil, repeats: true)
}
#objc func updateCounter() {
counter += 1
timeSinceRequest.text = String(counter)
}
#IBAction func acceptButtonTapped(_ sender: Any) {
//TODO: stop timer, make call to Firebase updating record status
}
}
Here's an example of the inconsistent counting. These two cells were loaded in at the same time, yet after about 6 minutes the second timer is behind by about 2 minutes.

Answer 1

Many thanks to #DanielStorm and #Paulw11 for the insights which led to this answer.
All logic for the timer has been moved out of the cell and into the primary ViewController.
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let customerRequest = activeRequestsArray[indexPath.row]
let cell = tableView.dequeueReusableCell(withIdentifier: "newRequestCell") as! newRequestCell
cell.setRequestCell(customerRequest: customerRequest)
let requestTime = NSDate(timeIntervalSince1970: TimeInterval(activeRequestsArray[indexPath.row].requestTimestamp/1000))
let calendar = Calendar.current
let timeComponents = calendar.dateComponents([.hour, .minute, .second], from: requestTime as Date)
let nowComponents = calendar.dateComponents([.hour, .minute, .second], from: Date())
let difference = calendar.dateComponents([.second], from: timeComponents, to: nowComponents).second!
cell.timeSinceRequest.text = String(difference)
cell.buttonPressed = {
print(indexPath.row)
//TODO: delete cell on button tap
}
return cell
}