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Decade
Find which decade, century and millennium any year belongs to.
Enter any year and this tool tells you exactly which decade, century and millennium it belongs to, along with the correct start and end years for each — accounting for the fact that centuries and millennia don't start on a round number the way most people assume.
Colloquially, people call the years 1900-1999 'the 1900s' and treat that as interchangeable with 'the 20th century', but strictly speaking the 20th century runs from 1901 to 2000, not 1900 to 1999. This is because there was no 'year zero' in the calendar system in common use — the 1st century ran from year 1 to year 100, which means every subsequent century is offset by one year from the round-number grouping people casually use. This tool calculates centuries using the strict definition, while noting the common informal usage for clarity.
Decades are more straightforward and less contested than centuries: a decade is simply a run of 10 consecutive years, and it's universally accepted in casual usage that 'the 2020s' means 2020 through 2029, aligned with the round-number decade rather than an offset boundary. This tool follows that common convention for decades while applying the strict definition for centuries and millennia.
The debate over whether a new century (or millennium) starts on a year ending in 00 or 01 famously played out at the turn of the year 2000, when celebrations happened worldwide, even though astronomers and calendar purists pointed out that the 21st century and 3rd millennium technically didn't begin until January 1, 2001. This tool gives you both the strict calculation and enough context to know why the popular and technical answers sometimes differ.
For any year you enter, the result shows the decade using the common round-number convention, alongside the century and millennium using the strict, no-year-zero definition, each with its correct start and end year spelled out. That way you get the answer most people expect for decades, and the technically accurate answer for centuries and millennia, without having to remember the offset rule yourself.
Yes, by strict calendar convention, since the calendar system in common use has no year zero — the 1st century ran from year 1 to year 100, meaning the 21st century runs from 2001 to 2100.
By the strict definition, yes — the actual 3rd millennium began on January 1, 2001. The 2000 celebrations followed the popular, round-number convention rather than the technical one, which is an entirely reasonable way to mark the occasion, just not the astronomically precise boundary.
No, decades are conventionally counted using round numbers, so 'the 1990s' means 1990-1999 exactly, without the same one-year offset that applies to centuries and millennia.
2020 through 2029 inclusive, following the standard round-number decade convention used throughout this tool.
Yes, the calculation applies the same consistent rule to any year, whether recent or centuries in the past.