Calculating days between dates comes up constantly — project deadlines, notice periods, contract lengths, anniversaries. The calculation is straightforward once you understand the method.
The Basic Method
Convert each date to a day number from a fixed reference point, then subtract. Most programming languages and spreadsheets do this automatically. For manual calculation, count the days remaining in the first month, add full months in between, then add days in the final month.
Example: Days from 15 March to 28 June. March remaining: 31 − 15 = 16 days. April: 30 days. May: 31 days. June: 28 days. Total: 16 + 30 + 31 + 28 = 105 days.
Leap Years
A leap year occurs every 4 years (year divisible by 4), except century years which are only leap years if divisible by 400. So 2000 was a leap year; 1900 was not. 2024, 2028, 2032 are upcoming leap years. Calculations spanning February of a leap year have one extra day compared to the same span in a non-leap year.
Business Days vs Calendar Days
Calendar days count every day including weekends. Business days (working days) exclude Saturdays and Sundays and typically also exclude public holidays. For legal and commercial purposes, notice periods and contract deadlines usually count in business or working days. "5 business days" starting Monday = Monday through Friday = arrives the following Monday.
Common Date Calculation Uses
- Employment notice periods (typically 1–4 weeks)
- Warranty and returns periods (30, 60, 90 days from purchase)
- Loan and mortgage terms
- Tax deadlines and filing periods
- Pregnancy weeks (days from LMP)
- Project timelines and milestones
Converting Days to Weeks and Months
Days to weeks: divide by 7. Days to months: divide by 30.44 (average days per month accounting for different month lengths). For exact month counting, count complete calendar months rather than using an average — this matters for contractual calculations.
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