I'll pay 100 robux to the solution!

[JaylinTooGoated (Jaylin)]

Now we can all rest stress-free! I can’t thank you guys enough man and I apologize if I came off as rude and/or annoying! I’m just here to learn!

[T3_MasterGamer (NotReal)]

i finally fixed the bug that another player can change your timer

Server:

local players = game:GetService("Players")

local dataStoreService = game:GetService("DataStoreService")
local dataStore = dataStoreService:GetDataStore("PersonalBest")

local replicatedStorage = game:GetService("ReplicatedStorage")
local bestTime = replicatedStorage:WaitForChild("BestTime")

local function playerAdded(plr)
	local personalBest = Instance.new("NumberValue")
	personalBest.Name = "PersonalBest"
	personalBest.Value = dataStore:GetAsync("Best_" .. plr.UserId) or math.huge
	personalBest.Parent = plr

	local currentTime = Instance.new("NumberValue")
	currentTime.Name = "CurrentTime"
	currentTime.Value = 0
	currentTime.Parent = plr

	local startTime = Instance.new("NumberValue")
	startTime.Name = "StartTime"
	startTime.Value = 0
	startTime.Parent = plr

	local timerStopped = Instance.new("BoolValue")
	timerStopped.Name = "TimerStopped"
	timerStopped.Value = true
	timerStopped.Parent = plr

	personalBest:GetPropertyChangedSignal("Value"):Connect(function()
		personalBest.Value = math.clamp(personalBest.Value, 1/math.huge, math.huge)
	end)

	while true do
		if timerStopped.Value == false then
			currentTime.Value = tick() - startTime.Value
		end
		task.wait()
	end
end

for _, plr in ipairs(players:GetPlayers()) do
	task.spawn(playerAdded, plr)
end

players.PlayerAdded:Connect(playerAdded)

players.PlayerRemoving:Connect(function(plr)
	dataStore:SetAsync("Best_" .. plr.UserId, plr.PersonalBest.Value)
end)

game:BindToClose(function()
	for _, plr in ipairs(players:GetPlayers()) do
		dataStore:SetAsync("Best_" .. plr.UserId, plr.PersonalBest.Value)
	end
end)

local startTimer = workspace.StartTimer
local endTimer = workspace.EndTimer

startTimer.Touched:Connect(function(part)
	local char = part.Parent
	local hum = char:FindFirstChildOfClass("Humanoid")
	local plr = players:GetPlayerFromCharacter(char)

	if char and plr and hum.Health > 0 and plr.TimerStopped.Value == true then
		plr.CurrentTime.Value = 0
		plr.StartTime.Value = tick()
		plr.TimerStopped.Value = false
	end
end)

endTimer.Touched:Connect(function(part)
	local char = part.Parent
	local hum = char:FindFirstChildOfClass("Humanoid")
	local plr = players:GetPlayerFromCharacter(char)

	if char and plr and hum.Health > 0 then
		plr.TimerStopped.Value = true
		if plr.CurrentTime.Value < plr.PersonalBest.Value then
			plr.PersonalBest.Value = plr.CurrentTime.Value
		end
	end
end)

Client:

local Players = game:GetService("Players")
local player = Players.LocalPlayer
local currentTime = player:WaitForChild("CurrentTime")
local personalBest = player:WaitForChild("PersonalBest")
local replicatedStorage = game:GetService("ReplicatedStorage")

local function secondsToMinSecMs(seconds)
	local minutes = math.floor(seconds / 60)
	local remainingSeconds = seconds % 60
	local secondsSplit = math.floor(remainingSeconds)
	local millisecondsSplit = math.floor((remainingSeconds - secondsSplit) * 100)

	return string.format("%02i:%02i.%02i", minutes, secondsSplit, millisecondsSplit)
end

local textLabel = script.Parent
textLabel.Text = "Timer: " .. secondsToMinSecMs(currentTime.Value) .. "\nPersonal Best: " .. secondsToMinSecMs(personalBest.Value)

while true do
	textLabel.Text = "Timer: " .. secondsToMinSecMs(currentTime.Value) .. "\nPersonal Best: " .. secondsToMinSecMs(personalBest.Value)
	task.wait()
end