Apart from not really seeing how they are very similar (Except the calendar view itself...which looks the same in every calendar app) it actually looks like Amie was around one year longer. At least based on public announcement timestamps I found. Doesn't really matter in the end, as they have very different ideas in the same field so there's nothing wrong with that.

- Amie, 2020-08-24: https://news.ycombinator.com/item?id=24263960

- Cron, 2021-11-18: https://cron.com/blog/2021-11-18-announcing-cron

I really hope Notion Calendar will build the "schedule todo" feature for its Calendar. That's the primary reason I used Amie.