Guides to time and dates
How to Calculate Age: Years, Months and DaysA clear guide to calculating exact age in years, months and days, including leap-day birthdays, with worked examples.How Is the Date of Easter Calculated?Why Easter moves every year: the rule, the history behind it, the earliest and latest dates, and a worked example of the computation.What Is an ISO Week Number? How Week Numbers WorkHow ISO 8601 week numbers are defined, why some years have 53 weeks, and how to find the week number for any date.Leap Years Explained: Why February Sometimes Has 29 DaysWhy we have leap years, the exact rule for which years count, and what it means for people born on February 29.The History of Daylight Saving TimeWho invented daylight saving time, when countries introduced it, why it was abolished in some places and what the debate looks like now.Public Holidays in Norway: Dates and How They Are CalculatedA complete list of Norwegian public holidays with dates for the next three years, and an explanation of which ones move with Easter.How Time Zones Work: UTC, Offsets and the Date LineA plain explanation of time zones: why they exist, how UTC offsets work, why some are half-hour offsets and where the date changes.UTC, GMT and Atomic Time: What Is the Difference?What UTC and GMT mean, how atomic clocks define the second, and why leap seconds exist and are being phased out.How to Count the Days Between Two DatesMethods for counting days between dates by hand, whether to include the last day, how leap years and months of different lengths matter, and how computers do it.How to Find the Day of the Week for Any DateA mental method, the doomsday rule, for finding which weekday any date falls on, with examples.Why Does an Hour Have 60 Minutes and a Day 24 Hours?The ancient history behind our units of time: Egyptian hours, Babylonian base 60, the seven-day week and the failed attempt at decimal time.Why Do Months Have Different Lengths? The Story of the CalendarHow the Roman calendar produced the month names and lengths we use, from Romulus to Julius Caesar and Pope Gregory.Midnight Sun and Polar Night: Why Daylight Changes So Much in the NorthWhy the sun never sets in summer and never rises in winter in the Arctic, with dates for Norwegian cities and an explanation of twilight.
Guides
Here you will find explanations of how time and the calendar work: why Easter moves, how week numbers are calculated, what makes a year a leap year, and how time zones and daylight saving time came about.
The guides are written for this site, with worked examples you can follow yourself. Each guide comes with a tool that does the arithmetic for you.