how give perk for 24 hours using datastore, developer products?

i was reading the roblox blog about developer products and one line says, "You could sell tickets to exclusive and secret parts of your map for a day." so i know it's possible to sell a perk that lasts for as long as i want it to last. I'd like to sell one of my developer products and make it remain for 24 hours, but i don't know enough to make that happen. right now i'm using the script i found in the wiki to sell the products.right now if the players reset or leave the server they will lose the perk. i just need someone to point me in the right direction to make it last for 24 hours.

`local MarketplaceService = game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("Purchases")

MarketplaceService.ProcessReceipt = function(receiptInfo) 

    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then
blahblahblah.....
end end

    local playerProductKey = "player_" .. receiptInfo.PlayerId .. receiptInfo.PurchaseId
    ds:IncrementAsync(playerProductKey, 1)


    -- tell ROBLOX that we have successfully handled the transaction
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end`

1 day later.... i've spent several hours trying to figure out a way to make this work but i am trying to do something beyond my scripting knowledge. if anyone can lend more help please do so. i will share the solution with the roblox community by posting it in the roblox forums. i think TheRobertDoges' idea with using utc time is a great part of the solution. i'm thinking i can record the time with purchaswe and then whenever players join the game it checks the datastore and if the purchase was made < 1000000 + (datastore time) then player would spawn with the perk. if purchase was mader > 1000000 + (datastore time) then nothing end. does that make any sense? the time format for utc is like 14digits and 1 day is like 1000000 or something.

would i do something like this?

`local MarketplaceService = game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("Purchases")

local perkstore = game:GetService("DataStoreService"):GetDataStore("Perks")
local buytime = Game:GetService("HttpService"):GetAsync("http://www.timeapi.org/utc/now?%25Q", true)




MarketplaceService.ProcessReceipt = function(receiptInfo) 

    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then
blahblahblah.....
end end

    local playerProductKey = "player_" .. receiptInfo.PlayerId .. receiptInfo.PurchaseId
    ds:IncrementAsync(playerProductKey, 1)

    local playerperkId = player.userId
       perkstore:SetAsync(playerperkId, buytime)


    -- tell ROBLOX that we have successfully handled the transaction
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end`

and then another script

local perkstore = game:GetService("DataStoreService"):GetDataStore("Perks")
local nowtime = Game:GetService("HttpService"):GetAsync("http://www.timeapi.org/utc/now?%25Q", true)

game.Players.PlayerAdded:connect(function(player)
player.CharacterAdded:connect(function(character)
    while not player.Character do wait() end
    local character = player.Character


local perktime = perkstore:GetAsync(player.userId)
if perktime  ~= nil then
   if  (perktime + 1000000) < nowtime then                    
     giveperk
else
  end end

   end)
end)