reply to Bob Doto at https://www.reddit.com/r/Zettelkasten/comments/14lcb4z/using_diaries_and_journals_as_source_material_for/
Ross Ashby kept his notes in notebooks/journals but he did cross-index them by topic using index cards. Rather than reference them by notebook (name/title/date) and page number, he kept a set of handwritten running page numbers across the entirety of his notebooks, so instead of Notebook 15 page 55, 1952 he'd simply write "3786" for page 3786. This can be seen on his index card for the indexed word "determinate" as an example.
For other examples, see: http://www.rossashby.info/journal/index/index.html
My own notebooks are usually titled by year and date spans along with page numbers, so I'll use those roughly as Bob describes. This has made it much easier to not need to move all my older notes into a card-based system, but still make them useable and referenceable.
For those with more explicit journaling, diary, or other writing habits, Ralph Waldo Emmerson makes an interesting example of practice as he maintained at least two commonplace books (a poetry-specific one and a general one) as well as a large set of writing journals where he experimented with writing before later publishing his work. Since there are extant (digitized and published copies) and large bodies of scholarship around them, they make an interesting case study of how his process worked and how others might imitate it.
On the diary front, of the historical examples I've seen floating around, only Roland Barthes had a significant practice of keeping his "diary" in index card form, a portion of which was published on October 12, 2010. Mourning Diary is a collection published for the first time from Roland Barthes' 330 index cards focusing on his mourning following the death of his mother in 1977.
Not as extensive, Vladimir Nabokov recorded a "diary" of sixty-four dreams on 118 index cards beginning on October 14, 1964 as an experiment. He was following the instructions of John Dunne, a British philosopher, in An Experiment with Time. The results were published by Princeton University Press in Insomniac Dreams: Experiments with Time by Vladimir Nabokov which was edited by Gennady Barabtarlo.
Presumably if one keeps a diary or journal in index card form in chronological order, they can simply reference it by date and either time or card X of Y, if there are multiple card entries for a single day. I keep a dated diary of sorts on index cards, though they rarely go past one card a day.