Something else must be editing the leaderstats since the script you made works fine for me, I also modified it a bit so its shorter. You should also be using task.wait() since wait() is going to get deprecated, more info on the announcement of the task library.
game.Players.PlayerAdded:Connect(function(player)
local leaderstats = Instance.new("Folder")
leaderstats.Name = "leaderstats"
leaderstats.Parent = player
local timeingame = Instance.new("IntValue")
timeingame.Name = "Time"
timeingame.Parent = leaderstats
local towers = Instance.new("IntValue")
towers.Name = "Towers Completed"
towers.Parent = leaderstats
while true do
task.wait(1)
timeingame.Value += 1
end
end)
(Both your original script and my modified version work for me)
I’ll also just throw in a screenshot.