Today is Sunday. Sunday is today.
Yesterday was Saturday. Saturday was yesterday.
Tomorrow is Monday. Monday is tomorrow.
This month is April. April is this month.
Last month was March. March was last month.
Next month is May. May is next month.
This year is 1997. 1997 is this year.
Last year was 1996. 1996 was last year.
Next year is 1998. 1998 is next year.
This decade is the Nineties. The Nineties is this decade.
Last decade was the Eighties. The Eighties was last decade.
Next decade is the Oughts. The Oughts is next decade.
This century is the 20th. The 20th is this century.
Last century was the 19th. The 19th was last century.
Next century is the 21st. The 21st is next century.
Now are you clear about when what is?