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Whether text can be interpreted as a date value highly depends on the locale worked in. "1/Oct/2018" is not a valid date format in en-US, where the date order is M/D/Y. Excel may be more lenient in what strings it accepts as dates, Calc is a bit more strict (though given there are two numbers with the second being 4 digits and one month name maybe could accept this as well). Working in an en-US locale any of "Oct/1/2018" or "1-Oct-2018" or "2018-Oct-1" would be accepted. Any non-English locale (where month names differ) would cease to work anyway.

To be locale (and application) independent the only safe bet is the ISO 8601 format "2018-10-01" (or "2018-10-1") as Mike already lined out.