What happened?
The universe began in a hot, dense state and has been expanding ever since. Stars, galaxies, planets, and people all belong to the story that followed.
How do we know?
Scientists study light from space, the expansion of the universe, and the oldest signals we can observe. NASA Space Place gives the universe's age as about 13.8 billion years.
What is still changing?
The calendar starts here by definition, but scientists keep improving measurements of the universe's age. If the accepted age changes, the scale can be recalculated.
Why it matters
Some time spans are too big to feel just by counting years. Starting the calendar here turns the universe's 13.8-billion-year story into a model we can compare: every later event gets a place in the same year.