Counting how long a player is on a part?

part = workspace.Part --Or the part you're working with

part.Touched:connect(function(hit)
    if not game.Players:GetPlayerFromCharacter(hit.Parent) then return end
    local char = hit.Parent

    local t1 = tick()

    part.TouchEnded:connect(function(hit)
        if hit.Parent == char then
            local t2 = tick()
            print(t2 - t1)
        end
    end)
end)

So, we first get the part we want to touch in a variable. We then wait for someone to step on that part. When someone steps on that part, we're making sure that it is in fact a ROBLOX user that's touching the part. If not, let's not do anything. Otherwise we're going to store his character model in a variable called "char". Here's where tick() comes in. What tick() does is it gets the time, in seconds, since January 1st, 1970. What we can do with that is, we can find differences in time. We store the first time in a variable called "t1". We then wait for the guy to get off the part. When he does, we're going to make sure that it's the same guy getting off the part. If it is, we're going to get the time again, and print out the difference in time.

This can be accomplished by using the Touched and TouchEnded event.

ObjectName.Touched:connect(function(Hit)
    return print(Hit.Name)
end)

ObjectName.TouchEnded:connect(function()
    return print("Off")
end)

But that's not it, We want to check if the Hitparameter is a player, We can do that by checking if the player has a Humanoid.

Object.Touched:connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") then
        return Hit
    end
end)

Next a loop, A loop is needed to continue the counting, 1,2,3,4,etc. In the Lua programming language there are many loops such as : for,while, and repeat loop, But in this case, a While loop will work, because the While loop continues looping till the current Condition is false or nil.

local count=0
Object.Touched:connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") then
        while wait(1) do
            count=count+1
        end
    end
end)

But if the player loses interaction, the timer continues, Thats where the TouchEnded event comes in handy. I'll also use a Boolean(true,false)

local count = 0
local PlayerOnPart = false

Object.Touched:connect(function(Hit)
    PlayerOnPart = true

    if Hit.Parent:FindFirstChild("Humanoid") then
        while wait(1) and  PlayerOnPart do
            count = count+1
        end
    end
end)

Object.TouchEnded:connect(function()
    count = 0
    PlayerOnPart = false
end)

I'm not 100% sure on how the code would look (I really do JS and C++); but if you start out by defining a variable and having that be for the time, then once the player touches the part, have it trigger a while loop that triggers a running sum on the time variable.