"Typing your code forces validations at the boundaries" was too strong because of course you can type your code without actually doing the validations, but you can structure your code such that that won't happen accidentally: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...

The idea is that checking should be the only way of making a value of the type. That prevents you from forgetting to check when you turn some broader type (say, string) into the more narrow one (date, in this case).