[jerfelix]

I'm curious about their statement that they automatically append a slash as soon as you type the month.

The instructions say "MM/YY", but do not indicate whether the month is required to have two digits, or just permitted to have two digits. My card says it expires 1/16 So when I key in 1, does a slash get appended? Probably not. Instead, it gets appended after 11. 11/6. now what?

[kijin]

11/6 may have been a valid expiration date before November 2006, but not anymore. So a smart algorithm could tell that you meant 1/16 rather than 11/6.

It even works if you put the year first. 16/1 is valid, and it's clear that 16 is the year. Meanwhile, 1/61 either contains an invalid month or is too far in the future to be a valid expiration date.

NearlyFreeSpeech.net [1] does it right. It accepts anything between 3 and 6 digits (MYY, MMYY, MYYYY, MM YYYY, YYYY/MM, etc) and only throws an error if parsing it results in an impossible date.

[1] https://blog.nearlyfreespeech.net/2015/05/13/new-payment-fea...