How to Calculate Your Exact Age and the Days Between Two Dates

Working out an exact age or the number of days between two dates sounds easy until you try it by hand. Months have 28 to 31 days, leap years add a day every four years (mostly), and it is easy to be off by one depending on whether you count the last day. Knowing the rules helps you check any result, whether it is for a visa form, a notice period, a pregnancy due date or a countdown.

How an exact age is calculated

An age in years, months and days counts whole calendar months from the date of birth first, then the days that are left. Someone born on 15 March 1990 is, on 25 October 2026, 36 years, 7 months and 10 days old: 36 birthdays, then 7 more whole months to 15 October, then 10 days.

The same person has been alive for 13,373 days. Total days are useful when a form asks for an age in days, or for milestones like your 10,000th day.

Days between two dates and the off-by-one problem

The difference between two dates normally counts the start date but not the end date. From 1 March to 1 April 2026 is 31 days, or exactly 1 month. If you need both days included, for example a hotel booking counted in days rather than nights or a leave request from Monday to Friday, add one: 32 days.

Business days are Monday to Friday. The same March period contains 22 of them. Public holidays differ by country and region, so subtract those yourself.

Leap years and month ends

• A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. 2000 was a leap year, 1900 was not, 2028 will be.
• If you were born on 29 February, your birthday falls on 28 February in non-leap years when ages are calculated this way.
• Adding a month to 31 January lands on the last day of February, because there is no 31 February. 31 January 2026 plus one month is 28 February 2026.

How to do it with QodeCat

The Age & Date Difference Calculator has three modes and shows results as soon as the dates are filled in.

1. Age: enter your date of birth. The second date is today by default, but you can change it to find out how old someone was or will be on any day. You get the exact age, your next birthday with the number of days until it, and the total months, weeks and days.
2. Between two dates: enter a start and an end date. Tick Include end date (+1 day) if both days should count, and Count business days only (Mon–Fri) if you want the working-day count as the main result. Total days, weeks and business days are always shown. If the end date is earlier, the dates are swapped for you.
3. Add / subtract: pick a date, choose add or subtract, then type an amount in days, weeks, months or years. Use it for deadlines like 90 days from today (25 October 2026 + 90 days is 23 January 2027) or a date 18 years ago.

Everything is calculated in your browser in whole days, so time zones and daylight saving changes never shift the result by a day.

Common mistakes

• Dividing total days by 365 to get years. Leap days make the result drift, and it gives no months or days.
• Forgetting whether the end date counts. Contracts and leave rules usually say which one they mean.
• Treating business days as all non-weekend days without checking for public holidays.
• Assuming every month is 30 days. A month counted from 31 January is shorter than one counted from 1 March.

Frequently asked questions

How many days until my birthday?

Use the Age mode. Under Next birthday you see the date, the weekday, the number of days until it and the age you will turn.

Can I calculate dates far in the past?

Yes, from the year 1 to 9999. The calculator uses today's calendar (the Gregorian calendar) for all dates, so very old historical dates recorded in the Julian calendar will differ.