Written using Flask. The app is open source and released under MIT License. I also wrote a blog post explaining the whole thing.
I’m not a fan of the cake day thing. It doesn’t matter at all which day you signed up. There is so many users that every day is the cake day of thousands of people. I think that mean it’s the opposite of being something special and worth caring about.
Your whole comment applies to actual birthdays also :P
Birthdays are typically celebrated with at most a few dozen other people, not, y'know, all of Reddit. ;)
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They(?) just gave a name to an otherwise established phenomenon in online communities. I distinctly remember being part of online communities in the early 2000s where members would at least notice that they had joined “N years ago today.”