Player Server Age

You could start an Int/Number Value to count upwards when the player joins, something like this:

game:GetService("Players").PlayerAdded:Connect(function(plr)
   local TimeValue = Instance.new("IntValue")
   TimeValue.Name = "PlayerServerAge"
   TimeValue.Parent = plr
end)

coroutine.wrap(function()
   while true do
       if TimeValue then
         TimeValue.Value += 1
      end
   end
end)()

And then using a .Changed event you could update the TextLabel locally, while showing the format using this resource.