What is the formal way of expressing 1990s?

How do you express the last decade of 20th century in formal written English? "1990s"? If the century is known from the context, can you simply say "the nineties"?

As in: "The involvement of US in Kosovo in the nineties"

Solution 1:

I would say that the nineties is formal enough for any context, since it does not detract from a scholarly air. It would not be preferable to write the 90s, though, since it is discouraged to use numerals in formal writing.

Solution 2:

‘The 1990s’ should do for most situations. ‘The nineties’ used to refer to the 1890s, and they were more spefically called ‘the Naughty Nineties’, but I suppose that’s too long ago now to risk any confusion.

Solution 3:

The nineteen nineties or The 1990s