How to Calculate the Days Between Two Dates
October 22, 2026
Learn how to count the days between two dates by hand and with a calculator: the simple rule, month lengths, leap years and the common mistakes to avoid.
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Practical guides to counting days, working with dates and reading the calendar.
How to Add Days to a Date
October 22, 2026
How to add days, weeks or months to a date and find the new date: the counting rule, month ends, weekdays and a quick way to check the answer.
How to Subtract Days from a Date
October 22, 2026
How to go back in time by days, weeks or months: find the date that was 30 or 90 days ago, with a worked example and the leap-year and month-end traps.
How Many Days Until a Date? Counting Down
October 22, 2026
How to find how many days, weeks and workdays are left until a date such as a birthday, holiday, deadline or trip, and how to read the countdown correctly.
Including the End Date in a Day Count
October 22, 2026
Why one day count can be off by one: how to decide whether the end date counts, with examples from hotels, leave, contracts and the rules that count the first day.
How to Count Workdays Between Two Dates
October 22, 2026
How to count Monday-to-Friday workdays between two dates by hand or with a calculator, including weekends, holidays, inclusive counting and common mistakes.
Business Days vs Calendar Days
October 22, 2026
What the difference between business days and calendar days is, how to convert between them, and why delivery and deadline promises depend on which one is used.
How to Calculate Your Exact Age
October 22, 2026
How to work out your exact age in years, months and days, how many days you have lived and when your next birthday is, with leap-day birthdays explained.
How Many Days Are in a Month and a Year?
October 22, 2026
How many days each month has, how long a year, quarter and half-year are, why February is short, and the shortcuts for remembering month lengths.
Leap Years Explained
October 22, 2026
What a leap year is, the rule that decides it, why 1900 was not one but 2000 was, how leap years affect date calculations and what leap-day birthdays mean.
What Is an ISO Week Number?
October 22, 2026
What an ISO week number is, how week 1 is defined, why some years have 53 weeks, and how ISO weeks differ from the week numbers used in the United States.
How to Find the Day of the Week for Any Date
October 22, 2026
How to find the weekday of any date by hand with the doomsday method, why weekdays repeat in cycles, and how to check a result with a calculator.
Adding Months to a Date: The Month-End Rule
October 22, 2026
How to add months to a date correctly: the same-day rule, the end-of-month case, why a month is not 30 days, and how spreadsheets and the law handle it.
How to Calculate Weeks Between Dates
October 22, 2026
How to count the weeks between two dates, how to show a result as weeks and days, and how elapsed weeks differ from ISO week numbers and calendar weeks.
How to Calculate a Pregnancy Due Date
October 22, 2026
How the estimated due date is calculated from the last period (40 weeks, 280 days), what Naegele’s rule is, and why most babies are born on a different day.
How to Count Deadlines and Notice Periods
October 22, 2026
How deadlines and notice periods are usually counted: the day of the event, the last day, weekends and holidays, with examples and why the rules vary.
Trial Periods and Cooling-Off Days
October 22, 2026
How to count free-trial days, return windows and legal cooling-off periods: when the clock starts, which day is the last and why the exact rule matters.
Date Differences in Excel and Google Sheets
October 22, 2026
How to calculate days, months, years, workdays and week numbers between dates in Excel and Google Sheets: the formulas, their quirks and a quick check.
Date Formats Around the World and ISO 8601
October 22, 2026
Why 04/05/2026 can mean two dates, how date formats differ between countries, and why the ISO 8601 format YYYY-MM-DD is the safe choice for sharing dates.
The Gregorian Calendar and Why Old Dates Differ
October 22, 2026
Why dates before the switch to the Gregorian calendar differ from modern ones, how countries changed at different times, and how a calculator treats old dates.