Guides

Plain-English explainers on how dates, day counts, and the calendar work — and which calculator to reach for.

What Is a Leap Year? The Rule, the Reason, and How to Check A leap year adds 29 February to keep the calendar in step with Earth's orbit. Learn the divisible-by-4, 100, and 400 rule, why it exists, and how to check any year. Read the guide How to Count the Days Between Two Dates (Inclusive vs Exclusive) The days between two dates depend on whether you count the start and end day. Learn the inclusive and exclusive methods, when to use each, with a worked example. Read the guide How Many Working Days Are in a Year? A typical year has about 260 to 262 weekdays, and roughly 250 working days once public holidays are removed. Learn how the total is worked out for any year. Read the guide Business Days vs Calendar Days: What's the Difference? Calendar days count every day; business days skip weekends and public holidays. Learn why "10 business days" is nearly two weeks, where each is used, and how to count them. Read the guide How Age Is Calculated (Years, Months, and Days) Your age is the completed years since birth, then the leftover whole months and days. Learn how month-ends and leap-day birthdays are handled, with clear examples. Read the guide Understanding ISO Week Numbers ISO 8601 week numbers run 1 to 52 or 53, start on Monday, and set week 1 as the week containing the year's first Thursday. Learn the rule and why January can be week 52. Read the guide How Many Days Are in a Year? (365 or 366) A common year has 365 days and a leap year has 366. Here is why the extra day exists, how to tell which kind of year it is, and the day counts that follow. Read the guide How Many Days Are in Each Month? Seven months have 31 days, four have 30, and February has 28 or 29. Here is the full list, the knuckle trick to remember it, and why February is the odd one out. Read the guide How Many Weeks Are in a Year? (52 or 53) Every year has 52 full weeks plus a day or two, and some years have 53 numbered ISO weeks. Here is why, and how to find the week number for any date. Read the guide How to Calculate Your Retirement Date Your retirement date is your date of birth plus your retirement age. Here is how to work it out, how to find your retirement age, and how to count down to it. Read the guide How to Calculate the Age Difference Between Two People The age difference between two people is the gap between their birth dates. Here is how to work it out in years, months, and days — and why the gap never changes. Read the guide How to Count Business Days Between Two Dates Business days are weekdays that are not public holidays. Here is how to count them between two dates, how to add them to a date, and why the answer varies by country. Read the guide