The Green Paper on EMU (European Monetary Union) states: By the end of the century, Europe will have a single currency. It will be strong and stable. This is the wish of its leaders and peoples in signing and then ratifying the Treaty on European Union. This claim will be judged TRUE if at least four countries of the European Union will have implemented mutually fixed exchange rates between their national currencies and the European currency (the euro) on or before the 30th May 2001, 1200 GMT. This claim may be judge TRUE before 30th May 2001, 1200 GMT if the above conditions have been met for a period of not less than 24 hours. Monetary Union with other countries' currencies (eg. The Netherlands adopting the German DM) will not be considered a valid reason to judge this claim TRUE. The name of the european currency is not important. It was formerly proposed as being the 'ecu' and is now known as the 'euro'. As long as the currency is not issued by a sole country (european or otherwise) and is issued in a european monetary union framework, it shall be considered as applicable for the purposes of the claim. Banknotes need not be printed for the claim to be judged TRUE.
References: EUROPA (at http://europa.eu.int/) Monetary Union (at http://europa.eu.int/en/agenda/emu/emumenu.html).
I have contacted the claim creator (1999-01-05) to get a definitive answer. He said the claim could be judged as soon as it happened (true) or 2001-05-30 (false).