Off-by-one trap: subtracting dates

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Subtracting one DATE from another in PostgreSQL returns an integer - the number of days between them. January 20 - January 15 = 5. A common off-by-one mistake: counting intermediate days (16, 17, 18, 19, 20) and getting 4, but SQL date subtraction counts from the start date itself, not the first day after it. In MySQL, use DATEDIFF(end_date, start_date) for the same result. To subtract timestamps and get an interval (with hours/minutes), subtract two TIMESTAMP values instead.