If you only know the year, you can enter 'yyyy (with the ' being important because it makes Excel think it's text not a number), e.g. '1987 (because if you just enter 1987 Excel will convert it to 9 June 1905!).
If you know the month and year, I would recommend uncertain dates in the form of yyyy-mm (which will be treated as text without needing the '), e.g. 2009-07 for July 2009.
If you can only guess the decade you could enter something like 198x or 200x, or 19xx or 20xx for century.
The important point is to start with the year and be consistent the formatting because the text 'dates' will sort automatically in any alphabetical list.