Age and Date Calculations: How to Calculate Time Between Dates
How to calculate exact age, days between dates, countdowns, and date math — with practical examples for common date calculations.
Calculating Exact Age
Exact age = Current date − Birth date. The complication: months have different lengths, and leap years exist. For a precise age:
- Start with birth year subtracted from current year
- Subtract 1 if the birthday hasn't occurred yet this year
- For days: count forward from the last birthday to today
Example: Born March 15, 1990, current date March 9, 2026: Age = 2026 − 1990 = 36, minus 1 (birthday hasn't happened yet in 2026) = 35 years old. Days until birthday: March 15 − March 9 = 6 days.
Days Between Dates
The Julian Day Number method is the most rigorous for computing days between arbitrary dates. For practical purposes: count the number of complete calendar days from the start date to the end date (not counting the start date, counting the end date — or vice versa, just be consistent).
Common Date Math Use Cases
- Contract dates: "Net 30" payment terms = 30 days from invoice date
- Pregnancy due date: LMP + 280 days
- Retirement countdown: Days until a target retirement date
- Lease expiration: Lease start + lease term in months
- Loan maturity: Loan start + term in months
Business Days vs. Calendar Days
Business days exclude weekends and federal holidays. When a contract specifies "5 business days," that's typically 7–9 calendar days. Courts, financial contracts, and government filings often distinguish between business and calendar days — read carefully and use appropriate calculations.