Time and dates: the calendar as a machine

Almost: century years are skipped unless divisible by 400. So 1900 was not a leap year, 2000 was, and 2100 will not be. The exception keeps the calendar aligned with a year that is slightly shorter than 365.25 days.

365 in an ordinary year, 366 in a leap year like 2024. Averaged over the Gregorian calendar's full 400-year cycle, a year is 365.2425 days — the fraction is why leap years exist.

Yes. In the ISO week system, a year that starts on a Thursday — or on a Wednesday in a leap year — contains 53 numbered weeks, and 2026 is one of them.

Working days conventionally means Mondays to Fridays, so a Friday deadline moved three calendar days lands on Monday — one working day later. Public holidays differ by country, so they are subtracted separately from your national list.

Because the Roman year originally ended in February, the month that absorbed the calendar's arithmetic shortfall — a shape later reforms preserved rather than fixed. The record shows the month lengths we use were settled close to their present form by the Julian reform and left alone since.

Historians trace the seven-day week to ancient Near Eastern tradition, with the seven classical 'planets' visible to the naked eye shaping the day names many languages still carry. Unlike the day or the year, the week matches no astronomical cycle — it survives because everyone agreed on it.

UTC is the modern reference standard, kept by atomic clocks and coordinated internationally; GMT is the older solar-time term that now mostly survives as the name of the winter time zone in Ireland and the UK. On an everyday clock they read the same — the difference is the machinery behind them.

In non-leap years, most people born on 29 February celebrate on 28 February or 1 March — convention, and in some places a legal definition for milestone ages, decides which. The date itself returns every four years: someone born on 29 February 2000 had seen just 6 actual leap-day birthdays by 2026.

Work out your exact age by counting completed calendar units in order: full years since birth, then full months, then whatever days are left over. Someone born on 7 July 1990 is exactly 36 years, 0 months and 0 days old on 7 July 2026, having lived 13,149 days. An age calculator does this in one step and handles 29 February birthdays exactly, so you never have to work out leap-year rules by hand.

Spend your leave days connecting holidays to weekends rather than sitting them in the middle of an ordinary week. A public holiday that falls on a Tuesday is the valuable case: one leave day taken on the Monday buys four consecutive days off. Which dates are worth targeting depends on your country's holiday calendar and which weekday each holiday lands on that year, so the best plays change annually. Because the calendar is fixed and known in advance, an optimiser can search every combination of leave days deterministically and find the pattern that returns the most days off for the fewest leave days used.

A bridge day is a single day of annual leave taken between a public holiday and a weekend, joining the two into one unbroken run of days off. It works because it removes the one working day that would otherwise split the break in two. Bridge days are the most efficient way to spend annual leave: a public holiday on a Tuesday or Thursday means one leave day can buy four consecutive days off instead of just a long weekend. Which dates qualify depends on where the public holiday falls in a given year, so the same holiday can offer a bridge day one year and not the next.