How the difference is counted
By default the count excludes the start date's end and the end date itself, the way a contract or loan counts "90 days from today". Tick "Include the end date" to count both ends, as you would when counting the days of a trip. Months are counted as calendar months: January 31 to February 28 is 28 days, not a full month.
Working days are Monday to Friday. With holidays turned on, the 11 US federal holidays are skipped, using the observed date when a holiday falls on a weekend (a Saturday holiday is observed on the Friday, a Sunday holiday on the Monday).
Days that matter for money
Interest on many loans and savings accounts accrues daily, so the exact number of days changes what you pay or earn. Credit card grace periods, payment due dates, the 60-day window for a 401(k) rollover and the three-day right to cancel some US loans are all counted in days. See the simple interest calculator for day-count interest.