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.