Help saving players time while offline?

I have it so when a player is moderated (banned) in the game, it sets a value in there player to the amount of seconds they have been banned for from my game, but I can't get it to count down. I don't want it to count down in game, cause if they try rejoining they will get kicked straight away. I have it so the seconds is timed by 60*60*24 to convert it to days, so if they are banned for 3 days, then the value would be 86400 I want this to countdown so even if the player isn't online it still counts down. I'm guessing it has something to do with os.time() but it's really confusing and the wiki doesn't explain it well.

moderationEvent.OnServerEvent:connect(function(player, ...)
    if player then      
        local tuple = {...}     
        local playerDataFolder = player:WaitForChild("PlayerDataFolder")
        if tuple[1] == "Banned" then    
            mydata.banned = true
            playerDataFolder.Banned.Value = mydata.banned
            mydata.bannedtime = tuple[2]*60*60*24
            playerDataFolder.Banned.BannedTime.Value = mydata.bannedtime
            -- I believe the countdown should go here --
        end
    end
end)

Do not worry that I didn't show what the event, playerdata, etc. are. That is not important here and there are no bugs. It's just trying to count down the time while the players gone. So basically I want the BannedTime.Value to count down while the player is not on the server.

Any help is appreciated.

EDIT

game.Players.PlayerAdded:connect(function(player)   
    local banned = Instance.new("BoolValue", playerDataFolder)
    banned.Name = "Banned"
    banned.Value = mydata.banned

        local bannedTime = Instance.new("IntValue", banned)
        bannedTime.Name = "BannedTime"
        bannedTime.Value = mydata.bannedtime


    local time = bannedTime.Value
    local timeBanned = moderationData:GetAsync(player.userId)
    local currentTick = tick()

    if (currentTick-timeBanned)>= time then
        --unban the player
    end
end)

moderationEvent.OnServerEvent:connect(function(player, ...)
    if player then      
        local tuple = {...}     
        local playerDataFolder = player:WaitForChild("PlayerDataFolder")
        if tuple[1] == "Banned" then    
            mydata.banned = true
            playerDataFolder.Banned.Value = mydata.banned
            mydata.bannedtime = tuple[2]*60*60*24
            playerDataFolder.Banned.BannedTime.Value = mydata.bannedtime
            moderationData:GetAsync(player.userId)
        end
    end
end)

Is this it? Or should I have a datastore that stores the tick()??