Counting days between dates
The simplest measure is total days: set both dates to midnight, subtract, and divide by the number of milliseconds in a day. By default the end date is not counted, so January 1 to January 3 is 2 days. That is the right convention for durations, ages and interest calculations. Check "include the end date" and the tool adds one, giving 3 days, which is how you count nights in a hotel bill turned into days, the length of a rental period, or the number of days you were on vacation when both the first and last day count.
The default range is today through 30 days from now. If you enter the dates backward, the calculator swaps them and notes that it did, so you never get a negative result. Spans up to 300 years are supported, which covers genealogy as well as business use.
Months and years are calendar-based
"Months and days" and "years, months, days" use real calendar months rather than a fixed 30-day month. The method counts whole months first: if the end day of the month is earlier than the start day, one month has not yet completed. Then it adds those months to the start date to get an anchor, and the days from the anchor to the end date are the leftover days. So January 31, 2026 to March 1, 2026 is 1 month and 1 day (January 31 plus one month lands on February 28, then one more day), while the total-days line says 29. This matches how leases, loan terms and ages are normally stated.
The "equivalent" line is a rough conversion using 7 days per week, 30.44 days per month and 365.25 days per year. It is useful for comparing periods of different lengths, but for anything contractual use the calendar-month figure.
Business days
The business-day count walks through every date in the range and counts Mondays through Fridays. It deliberately does not remove federal holidays such as Thanksgiving, Christmas, Memorial Day or Juneteenth, because holiday schedules vary by employer, state and industry, and banks, courts and schools each have their own calendars. To get working days for a specific purpose, subtract the holidays that fall on weekdays in your range. There are eleven federal holidays a year, so a full calendar year has roughly 250 business days.
Common uses: figuring how many days a payment is past due, the length of a notice period, days until a filing deadline, weeks of pregnancy from a known date, or how many working days are left in a quarter. For counting forward from a date rather than between two dates, use the Add or Subtract Days calculator.