Another possibility is to give two years:
FROM 1990 TO 2000
BET 1990 AND 2000
Though this is not recommended in the Gedcom-Standard (as isn't the "o|o"-Solution). Ahnenblatt will export both solutions correct, but you don't know what the importing software will do until you test it.
Marcus _________________ Fragen und Antworten rund um Ahnenblatt (Knowledge Base)