"在二十世纪90年代" 是 in the 1990s. 二十世纪翻成英语变成了19

in the 21th century
在年份中**世纪要在本世纪后－1
但直接读的时候不变 21世纪还是21th century

In the 21 first century is the right one,

In the 1990s referres the year between1990-1999

The 21st century referres to 2001-2100

It is hard to explain ,just remember it