I'm a heavy user of todotxt. For me, the greatest thing about keeping everything in a txt file is that it is very easy to build your own extensions with a combination of sh, awk, grep, etc.

For calendars, the problem is a bit more problematic, though. Yes, there are things like calendar[1] and remind[2], but nobody seems to use them nowadays and in the mobile world there is no app to process them. Do you know of any existing solutions for calendars that work like todotxt?

[1] https://man.netbsd.org/calendar.1

[2] https://linux.die.net/man/1/remind