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Calendar

If 3 March is a Monday, what day is 27 April? 100 days from Tuesday? Days between two dates, and leap years — with spans that grow from days to centuries.

  • Calculation
  • Timed
  • Endless levels

How it plays

Weekday arithmetic: whole weeks change nothing, so only the remainder on division by 7 matters. 100 days from a Tuesday is 100 = 7 × 14 + 2, two days on: Thursday.

From one date to another, count the days month by month; across years, each year is 365 days plus one for every 29 February crossed. Days between two dates are asked counting both ends, neither, or only the first — read which.

Leap years: every fourth year, except century years, which must divide by 400 — so 1900 was not and 2000 was. Spans grow without end. Dates are in the Gregorian calendar and never before 1600.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This trains calendar arithmetic — remainders mod 7, month lengths, leap years. It is practice at this, nothing wider.

Learn

How to get better at calendar: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Read what kind of question it is first: N days from a weekday, the weekday of a date, the days between two dates, or a leap year. Each has its own short method below.
  2. For a count of days, throw away whole weeks at once. Only the remainder on division by 7 moves the weekday.
  3. For two dates, count whole years first (365 each, plus one for every 29 February crossed), then the rest month by month: the days left in the first month, the full months, then the days into the last.
  4. Before you answer, re-read the direction (“ago” or “from now”) and, for days between, which ends are counted. Most wrong answers here are right sums pointed the wrong way.

Techniques and tricks

  • Remainder only

    Divide the number of days by 7 and keep the remainder. Count that many days on (or back) from the weekday you were given.

    e.g. 100 days from a Tuesday: 100 = 7 × 14 + 2, so two days on, Thursday.

  • Landmark multiples of 7

    Keep 70, 140, 210, 280, 350 and 364 in your head. Take the nearest one below the count away and you are left with a small remainder to finish by hand.

    e.g. 367 days: 367 − 364 = 3, so three days on.

  • One a year, two across a leap day

    365 is 52 weeks and 1 day. The same date a year later is one weekday on, or two if a 29 February falls in between.

    e.g. 3 March 2027 is a Wednesday; 29 February 2028 is crossed, so 3 March 2028 is a Friday.

  • Month shifts

    A 31-day month moves the weekday on 3, a 30-day month 2, February 0, or 1 in a leap year. The same day of the next month is that many days on.

    e.g. If 5 April is a Friday, 5 May is a Sunday: April has 30 days, 30 = 28 + 2.

  • Days left, then days into

    Across months, take the days left in the first month, add the full months between, then add the date in the last month.

    e.g. 3 March to 27 April: 28 left in March + 27 = 55 days; 55 = 7 × 7 + 6, so a Monday becomes a Sunday.

  • The leap-year test in two steps

    Does it divide by 4? If not, no. If it is a century year (ends in 00), it must also divide by 400.

    e.g. 1900: divides by 4, is a century, 19 is not a multiple of 4, so no. 2000 and 1600: yes. 1640: yes.

  • Fence posts for days between

    The plain difference counts the first day and not the last. Counting both days adds 1; counting neither takes 1 away.

    e.g. 16 April to 5 July is 80 days; not counting either day, 79.

As the levels climb

  • The first notch stretches the spans from 12 days to 42; after that the span and the clock take turns. Spans reach 87 days, then 146, then 217, and keep growing past a year, then past several years, without end.
  • The clock closes in steps of 85%, 72% and 62% of the base time, reached by level 6. From then on only the spans grow, so expect the extra seconds a longer span earns to matter more.
  • Once spans pass a year, stop walking month by month. Jump whole years at 365 days each, count the 29 Februarys you cross, then walk only the leftover months.
  • Multi-year weekday questions become a count of shifts: one per year plus one per leap day crossed, then the month walk, all reduced mod 7 at the end.

Traps to avoid

  • One day out either side, often from counting the given day as day one. Count the first step as the day after the given one.
  • The right remainder counted the wrong way: two days back from a Tuesday given when the question said “from now”. Say the direction aloud before you count.
  • The century years. 1700, 1800 and 1900 were not leap years; 1600 and 2000 were. The leap-year questions are drawn to land on these edges often.
  • In days between, the answers one and two apart are the counting-both, counting-neither and plain-difference answers. Do the difference first, then adjust by exactly the rule the question names.
  • A missed 29 February across years. Before adding 365 per year, list the leap years you cross, and check whether the span actually includes their 29 February.

How to practise

  • Use the Questions setting to drill one kind at a time; the weekday of a date and days between share the same month walk, so practise those two together.
  • Switch Answer by to “Typing the day or number” once choosing feels easy. With options you can rule out by the remainder alone; typing makes you do the whole count.
  • Learn the month lengths and the month shifts (3, 0, 3, 2, 3, 2, 3, 3, 2, 3, 2, 3 from January) until you can say them without thinking; the long spans are built from them.
  • When a wrong answer shows its working, check which line differs from yours: the year count, the leap days or the month walk. That tells you which step to slow down on next time.

Levels

Each question has a clock that comes from the work it asks for. There is no last level: at least one dial turns without end. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

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