• Subject: Re: Credit card expiry date validation
• From: Kaj Eijlers <bizziboi@...>
• Date: Mon, 15 Jun 2020 13:26:39 -0700

Calculate the delta between y1 and y2, if it's larger than 50 you want (100-delta)

99-2 = 97
100-97 = 3.

I think?

On Mon, Jun 15, 2020 at 1:15 PM Wilmar Pérez <darioperezb@gmail.com> wrote:
Hi all,

I have an application collecting credit card data. Before sending the information out to the payment entity I am trying to make sure the information entered is, at least, valid. I already worked out the card number and cvv numbers but I am not so sure about the expiry date. The format I get the info is MMYY. So what I am doing is:

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-- Simple function to get current date and times function getdatetime(tz) local tz = tz or 'America/New_York'; local luatz = require 'luatz'; local function ts2tt(ts) return luatz.timetable.new_from_timestamp(ts); end local utcnow = luatz.time(); local time_zone = luatz.get_tz(tz); local datetime_raw = tostring(ts2tt(time_zone:localise(utcnow))); local year, month, day, hour, min, sec, time_reminder = string.match(datetime_raw, "^(%d%d%d%d)%-(%d%d)%-(%d%d)[Tt](%d%d%.?%d*):(%d%d):(%d%d)()"); return year, month, day, hour, min, sec; end local current_year, current_month = getdatetime() -- Get current year/Month local card_expiry_date = 'YYMM'; -- In the app this actually get a value eg: 2204, 2301, 2010, etc. local card_exp_year = string.sub(card_expiry_date , 3, 4) local card_exp_month = string.sub(card_expiry_date , 1, 2) -- Extract the last two digits of the Year current_year = string.sub(current_year , 3, 4) -- Check month is valid if(card_exp_month < '01' or card_exp_month > '12')then print("This is not a valid month") else -- Check date is this month or after if((card_exp_year < current_year) or (card_exp_year == current_year and card_exp_month < current_month))then print("Date cannot be before this month.") else print("All is good.") end end

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I do not know if this is the most elegant solution but it works. However it has a huge bug: it will fail at the end of the century. Since I only know the last two digits of the expiry date year, if a card expires in 2102 for instance and we were in 2099 my logic would wrongly reject the date (02 is less than 99).

I am very aware that me an my simple app will likely not be around by then but it bugs me to leave it like this.

Can anyone please suggest a proper way to do this validation?

Thank you!

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Wilmar Pérez