I read an article a few weeks back about Ronald Reagan's diaries. During his presidency (1981 through 1989), he wrote a diary entry every day, in long hand. Excerpts were published in Vanity Fair, and the writing reveals a terse prose. The entry after his attempted assassination in March 1981: "Getting shot hurts."
It must be hard to write a diary when you know that scholars will one day want to read them. His diary entries were source material for his memoir, and for the press during the Iran-Contra arms sale scandal.
I've always considered diaries and journals to be private matters. I keep a diary, and I once thought I was writing it for some "future reader." Now I well understand that I'm writing only for my "future self." As such, my diary prose is often long-winded. Unlike Reagan, who didn't spell out expletives (e.g. he used "h--l" and "d---"), I use them quite liberally, and in all caps (my diaries are electronic) with plenty of exclamation points.
I'm not the president, so I can rest assured that my diary will disappear when I eventually shake off my mortal coil. But until then, I'll need to keep writing in it because "the future me" is still interested.