Diary writing is something that you write to yourself, so it is classified under informal writing. But, from an examination perspective, you cannot be too informal. So, it is "formally informal".
The simple format of diary writing is to draw a box, write the date and day on the top right corner, write "dear diary," below that on the left hand side, write your experience and resolutions in about 2-3 paragraphs, and finally end it with a "good bye" or "good night" or "more later..." along with your name after it.
Hope it helps :)
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