Enter any year to find every other year that shares its exact calendar layout. Explore why leap years exist, and how we ended up shifting our clocks twice a year.
Enter a year above to find its calendar twins
Think of the calendar as a clock that runs slightly too slow. Earth doesn't orbit the sun in exactly 365 days — it takes about 365 days, 5 hours, 48 minutes, and 45 seconds.
Those extra hours build up. If we ignored them, our calendar would desyncWhen the dates on your calendar no longer match the physical seasons. Without leap years, January would eventually move into summer.. After a century of ignoring this, your calendar would be nearly a month behind the actual position of the sun.
Simply adding a leap day every 4 years adds too much time. To fix this over-correction, we skip leap years on centuries — like 1900 — unless they are divisible by 400, like 2000. This keeps our human clock in step with the cosmos.
We are living in a loop. Since there are only 7 days to start a year and two types of years — CommonThe standard 365-day year. and LeapThe longer 366-day year, with February 29th. — you only ever experience 14 different calendar layouts. You've seen them all before.
For most years, the calendar fully repeats on a 28-year cycle — the least common multiple of the 4-year leap cycle and the 7-day week. Some years sync up sooner (in 6 or 11 years), but every calendar eventually loops back within 28 years.
DST is a collective agreement to shift our clocks so we have more usable sunlight in the evening during summer. We're not actually saving light — we're just rearranging when we use it.
While Ben Franklin joked about waking up earlier to save on candles, the real push came from George Hudson, an entomologist. He wanted more daylight after his 9-to-5 job so he could spend his evenings collecting insects.
Germany officially adopted the shift in 1916 during WWI. The goal wasn't to help farmers — who actually hated the change — but to conserve coal. If people were active while the sun was up, they didn't need to burn fuel for artificial light.
In the US, we "Spring Forward" on the second Sunday in March and "Fall Back" on the first Sunday in November. Most of the world — including roughly 60% of people — doesn't observe DST at all.