The best format for dates in a resume is the “month year” combination, e.g., “October 2020”. Usually, there is no need to include the day of the month as well. If you want to save up space you can shorten the month to its three-letter abbreviation, e.g., “Oct 2020”, or the year to its two-digits equivalent, e.g. “Oct ‘20”. You should separate the start date and the end date with a dash (“-”) or “to” and if you’re still working in a position, put “Present” or “to date” for the end date.