The one decision that changes the answer: is the end date counted?
Ask two people how many days there are from Monday to Friday and you can get two answers, 4 and 5. Both are right. They are answering different questions:
- Elapsed time (the default here).How much time passes between the two moments? From Monday morning to Friday morning is 4 days. Countdowns, ages, interest periods and “days until” questions all work this way.
- Days touched (tick “Include the end date”). How many calendar days does the period cover? Monday through Friday covers 5. Leave requests, event durations, medication courses and anything described as “from and including” to “up to and including” work this way.
When a document gives a deadline in days, read how it counts them. Legal and banking rules often say outright that the first day is excluded and the last day included, which gives the same number as elapsed time. If it doesn’t say, the safer reading is usually the one that gives you less time.
Why the months-and-days figure is not just total days divided by 30
Months are 28, 29, 30 or 31 days long, so “how many months” depends on which months you cross. The calculator counts the way a person reading a calendar would. It moves forward one whole month at a time until another month would overshoot, and then counts the remaining days. When the start day doesn’t exist in a shorter month, it stops at that month’s last day. So January 31 plus one month is February 28 (or 29 in a leap year), not March 3.
One surprising result: January 31 to February 28 and January 28 to February 28 are both “1 month”, even though they are 28 and 31 days long. Where precision matters, such as an interest calculation or a notice period, use the total-days figure. Contracts that mean days should say days.
Counting by hand, and the shortcuts that work
Without a calculator, break the span at month boundaries. From March 10 to June 2: 21 days remain in March (31 − 10), then 30 in April, 31 in May and 2 into June, which is 84 days in total. Two shortcuts help:
- Same day, later month: the answer is the sum of the lengths of the months you leave. From March 10 to June 10 you leave March (31), April (30) and May (31), so it is 92 days.
- Weeks are always 7 days. A date exactly 13 weeks ahead is 91 days ahead and falls on the same weekday. That is why the weeks-and-days figure is handy for scheduling.
The knuckle rule gives month lengths: make a fist, and each knuckle is a 31-day month and each dip between them a shorter one. July and August are both knuckles, which is why they are both 31 days.
Working days, weekends and public holidays
The business days figure counts Monday to Friday in the same range and follows the same end-date setting. Choose a holiday calendar and it also takes out public holidays that fall on weekdays. The calendars are US federal holidays, using the observed date when a holiday falls on a weekend, and England & Wales bank holidays with their substitute days. A 4-week span usually has 20 working days, and a December with Christmas and New Year in it often has 2 or 3 fewer. To find a deadline (“10 business days from today”) or see working days month by month, use the business days calculator. To move a date forward or back by a set number of days, weeks or months, use the date calculator.
Time zones and daylight saving
This counter works in whole calendar days, so a daylight-saving change can’t throw it off. The spring night has 23 hours and the autumn night 25, but each is still one day. “Today” is taken from your own device, so someone in Sydney and someone in San Francisco will see different starting dates for a few hours each day. For a precise age in years, months and days (for example for an eligibility cut-off), the age calculator uses the same calendar rules.