I do not know any Latin at all. Could you please help me to solve the following problem? I am wondering what would be a correct equivalent for "ex libris" used in books to change the meaning from "from books" to "from diaries" or "from calendars" ? Something like "Ex diariis"?
Ex diariis is correct, although diarium usually means "daily ration/allowance" and seems to have been used in the sense of a "diary" only rarely.
A more classical expression for "diary, journal" may be commentarii diurni (daily notebook), so you could say: ex commentariis diurnis (or diurnis commentariis, whichever you prefer).