[MEMBERSHIP Style DevProduct] - Where can I start from?

This is a good question. The few constraints we would need to consider are that people can buy the membership repeatedly while still having a membership. We would not want to cut their time short, so we need some way of adding to when it will expire.

I would implement this with the following tools.

• timestamps from os.time a timestamp is a way of expressing what the date and time is at a particular point. For example, when I am writing this question os.time returns 1513446506. (This means that many seconds have passed since the 1970 January 1st Epoch that you can read about here) By getting the difference with the next os.time that we call, we can track how many seconds have passed.

• the process receipt callback from the MarketPlace Service (example is here)

• data stores to track the timestamps from two bullets above.

The idea is that you track when membership will expire in terms of timestamps. You renew this expiration time in a data store when purchases are made.

I would implement it with something like the following and I assume that you can fill in the rest.

constant = 86400 -- membership time in seconds (because remember timestamps are in seconds)

processReceipt = function(info) -- the callback.
    pcall(function() -- remember to check for web service errors from Roblox, a
                -- nd maybe do more error handling than I do here.
        expirationtimestamp = mydatastore:GetAsync(info.PlayerId) -- old timestamp
        currenttime = os.time() -- we get a new timestamp
        if expirationtimestamp > currenttime then
            mydatastore:SetAsync(info.PlayerId, expirationtimestamp + constant) -- we add to the expiration time.
        else
            mydatastore:SetAsync(info.PlayerId, currenttime + constant)
        end
    end) 
    return Enum.ProductPurchaseDecision.PurchaseGranted 
end

And then when a player logs in to make sure that they have membership you would do.

local player -- Assuming you know who the player is.
expiration = mydatastore:GetAsync(player.UserId)
currenttime = os.time()

if currenttime < expiration then
 -- they are still a member
end