How to make a timer that runs server side and affects clients?

There's a few ways you could do this

Option 1: Have an int value the server updates to count down, which is accessible by the client so long as it's somewhere it can access like workspace.

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Option 2: Use a remote function to return the time left when the client requests it;

Server script:

local timeLeft = 100

spawn(function()
    for i = 100, 1, -1 do
        wait(1)
        timeLeft = i
    end
end)

function GetTime()
    return timeLeft
end

script.RemoteFunction.OnServerInvoke = GetTime

Local script:

while wait(1) do
    print(workspace.GetTime.RemoteFunction:InvokeServer())
end

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Option 3: Have the server send the time to all the clients every 1 second;

Server script:

local timeLeft = 100

spawn(function()
    for i = 100, 1, -1 do
        wait(1)
        timeLeft = i

        script.RemoteEvent:FireAllClients(timeLeft)
    end
end)

Local script:

workspace.GetTime.RemoteEvent.OnClientEvent:Connect(function(timeLeft)
    print(timeLeft)
end)