Calculating Ages and Date Differences Without Off-by-One Errors
Contracts, notice periods and visa deadlines all hinge on date arithmetic — and mental date maths is wrong more often than people think.
Date arithmetic looks trivial until a deadline is missed by a day. Three rules cover almost everything.
Rule 1: decide whether counting is inclusive
"Within 30 days of receipt" usually excludes the day of receipt and includes the final day. "Five working days' notice" excludes both weekends and public holidays. Read the definition in the document; do not assume. The date difference calculator gives the exact number of days between two dates, which you then adjust for the convention in force.
Rule 2: months are not 30 days
Adding "three months" to 30 November gives 28 or 29 February, not 30 February. Legal convention is normally the same day number in the target month, clamped to the last day if it does not exist. Adding 90 days instead gives a different date entirely — again, check which the contract specifies.
Rule 3: age is completed years
Age is the number of birthdays passed, not the year difference. Someone born in December 1990 is 34, not 35, in June 2026. The age calculator reports years, months and days, which matters for school-entry cutoffs, insurance bands and pension eligibility.
Leap years
A year is a leap year if divisible by 4, except centuries not divisible by 400. 2000 was; 1900 was not; 2100 will not be. Anyone born on 29 February legally has a birthday on either 28 February or 1 March depending on jurisdiction.
Practical uses
Notice periods, probation end dates, warranty expiry, visa validity, project milestones, and "how many days until launch". For recurring planning, count backwards from the deadline and add buffer in working days, not calendar days — a four-week task starting mid-December is not a four-week task.
FAQ
Does the calculator handle time zones? It works on calendar dates, which is the correct basis for contractual deadlines.
How are working days counted? Weekends are excluded; public holidays vary by country and must be subtracted manually.
Why does my age differ by a day from another tool? Some tools count the birth day itself; completed-years counting does not.
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