I'm not a lawyer so take this with a grain of salt, but it seems like there is a big gap between speech that is "directed to inciting or producing imminent lawless action" and just saying that you will do something.

Depending on context saying "I'm going to rob the bank X tomorrow" might also count as a threat?

But, in either case, writing this in a private diary could not be incitement or a threat because you are not communicating with anybody except yourself.