How to Count Days Between Two Dates: Inclusive vs Exclusive
Why the days between two dates can have more than one correct answer—and a reliable method for inclusive dates, leap years, business days, deadlines, and DST.
Time & Date Stories
Articles about calendars, clocks, time zones and the little stories behind the dates we use every day.
Why Does February Have Only 28 Days?
February is short because of a Roman superstition about even numbers, a vanishing month called Mercedonius, and 2,000 years of nobody daring to touch it. The real history — and why the famous Augustus story is a myth.
Is 12 PM Noon or Midnight? AM, PM and the 24-Hour Clock
By convention 12 PM is noon and 12 AM is midnight, but those are the two moments where the 12-hour clock breaks its own logic. Here is what AM and PM really mean, why they stumble, and how to write the time so nobody is confused.
Unix Time and the Year 2038 Problem
Unix time is a single number counting seconds since 1970, and on January 19, 2038 the 32-bit version runs out. Here is what that means, why it already breaks things, and how it gets fixed.
Why Recurring International Meetings Shift During Daylight Saving Time
A weekly 9:00 AM New York meeting does not stay at the same local time everywhere. This worked example shows the 2026 daylight saving gaps across London, Madrid, Los Angeles, Dubai, Tokyo, and Sydney.
Same Deadline, 10 Countries
We ran the same deadline rules through 10 national holiday calendars. A simple 10-business-day deadline spread from May 14 to May 19, and Christmas deadlines moved differently by country.
What Time Is It on the Moon?
Clocks on the Moon do not tick like clocks on Earth. Here is why NASA is building Coordinated Lunar Time, and why a few microseconds per day matter for navigation, safety, and the future of lunar life.
The Most Common Date and Time Bugs in Software
A practical guide to the mistakes that keep breaking software: naive datetimes, UTC confusion, DST gaps, date-only bugs, Unix timestamp units, week numbers, and more.
ISO Week Numbers vs US Week Numbers
ISO weeks start on Monday and make Week 1 the week with the first Thursday. US-style week numbers often start on Sunday and count the week containing January 1 as Week 1. Around New Year, those rules can disagree by a full week.
Why Business Days Are Not Universal
A business day sounds precise, but it is not a natural unit. Local weekends, public holidays, observed holidays, and legal definitions can all move the same deadline to a different date.
The IANA Time Zone Database: The Hidden Infrastructure Behind Your Clock
Behind every time conversion on modern devices sits a public database of political decisions, historical guesswork, naming rules, and last-minute updates. Here is how the IANA time zone database works and why so much software depends on it.
How to Write Dates and Times Clearly for International Teams
A practical guide to avoiding date-format, timezone, AM/PM, DST, and week-number mistakes when people work across countries.
Why the Week Starts on Sunday in Some Countries and Monday in Others
There is no single natural first day of the week. Sunday-first calendars, Monday-first workweeks, ISO week numbers, and regional settings all come from different layers of history, religion, labor, and software.
How Midnight Became the Start of the Day
A day did not always begin at midnight. Some cultures started it at sunset, others at sunrise, and astronomers once counted from noon. The modern midnight day is the result of law, clocks, astronomy, and the need for one clean boundary.
Why the US and Europe Change Clocks on Different Dates
For a few weeks every year, the time gap between North America and Europe quietly changes by an extra hour. Here is why the rules drift apart, what the mismatch actually does to meetings and deadlines, and why the two systems still have not been aligned.
The Leap Second Is Ending
Leap seconds keep atomic time close to Earth rotation, but they also create one of the strangest moments in civil time: 23:59:60. Here is why they exist, why computers dislike them, and why the world is preparing to stop using them by 2035.
Which Years Have 53 Weeks?
Most years stop at Week 52, but some have a Week 53. Under the ISO 8601 week-date system, that happens when January 1 falls on a Thursday, or when a leap year begins on a Wednesday. Here is the rule, the reason behind it, and the years when it occurs.
Why Does Easter Move Every Year?
Easter is not a random “floating holiday.” Its date comes from a centuries-old rule that combines Sunday, the spring equinox, and a tabular full moon. Here is how that rule works, why it can land anywhere from late March to late April, and why Orthodox Easter often falls on a different Sunday.
UTC vs GMT: What’s the Difference?
On any given day, UTC and GMT differ by less than a second — and yet they are not the same thing. One is a meridian on a hill in London, the other is the weighted average of hundreds of atomic clocks. Here is why we have both, what the difference really is, and the surprisingly narrow set of cases where it actually matters.
Why Do Months Have Different Lengths?
January has 31 days, February has 28 (or 29), April has 30. There is no mathematical pattern, no astronomical reason, and no logical system behind it. The answer involves an ancient Roman king, two power-hungry emperors, and a month that has been getting shortchanged for over 2,000 years.
Why Does a Week Have 7 Days?
Days come from Earth’s rotation, months from the Moon, years from the Sun. But the 7-day week has no astronomical basis at all — it is purely a human invention. From ancient Babylon to the failed French and Soviet experiments, the strange story of our most unnatural time unit.
How the International Date Line Works
Somewhere in the middle of the Pacific Ocean, today becomes tomorrow — or yesterday. The International Date Line is one of the strangest inventions in timekeeping: an invisible boundary where the calendar jumps forward or backward by a full day.
Why Some Countries Live in a Different Year
Not everyone on Earth agrees what year it is. While most of the world follows the Gregorian calendar, countries like Thailand, Iran, Ethiopia, and Israel each have their own calendar systems — and their own year numbers. The differences are far more than symbolic.
Daylight Saving Time: Who Invented It, Who Hates It, and Will It Ever End?
The twice-yearly ritual of moving clocks forward and back has been around for over a century. From a New Zealand entomologist’s 1895 idea to the EU’s ongoing abolition debate, here is the full story of daylight saving time — who started it, what it actually saves, and why so many people want it gone.
How NTP Keeps Billions of Clocks in Sync
Every phone, laptop, and server on Earth agrees on the time to within milliseconds. That is not an accident — it is the work of the Network Time Protocol, a system invented in 1985 that quietly keeps the modern world running.
Leap Years Explained: The Math Behind February 29
Earth takes 365.2422 days to orbit the sun. Here is the elegant rule the Gregorian calendar uses to stay aligned, why 2000 was a leap year but 1900 was not, and what leap seconds add on top.
ISO 8601 Explained: The International Date Format Every Developer Should Use
Why YYYY-MM-DD is unambiguous, how the standard handles weeks, durations, and intervals, and the reasons it powers everything from databases to JSON APIs.
How Time Zones Really Work: From Sandford Fleming to UTC
Before 1883, every town set its own clock by the sun. The story of how railways forced standardization, why we have 24 zones (and actually 38), and how UTC keeps the world in sync today.
The History of the Gregorian Calendar: Why October 1582 Lost 10 Days
How a 16th-century papal reform corrected a 1,600-year-old drift in the Julian calendar, why Protestant nations resisted it for centuries, and how it became the global civil standard.