Extract from the European Central Bank Article (http://ec.europa.eu/economy_finance/thematic_articles/article12891_en.htm):

"On 8 July 2008 the Council of the EU took the final formal decision allowing Slovakia to adopt the euro from 1st January 2009. The decision was celebrated with festivities on the roundabout Schuman in the heart of the European quarter in Brussels led by Commissioner Almunia.

The Council also irrevocably fixed the conversion rate for the Slovak koruna to the euro. Over the next 5 ½ months, the country will have to complete and finalise its practical preparations to ensure that the changeover to the euro goes as smoothly as it did in Slovenia, Malta and Cyprus.

Today's final decision of the Council followed the conclusions of the European Commission's convergence report of 7 May 2008 that Slovakia met the convergence 'Maastricht' criteria and qualified for the euro. The euro area will thus grow to 16 members on 1 January 2009. This is already the third expansion of the euro area from among the ten new Member States which joined the EU in 2004.

In addition to the formal final decision on the euro adoption in Slovakia, the Council, meeting in its configuration of finance ministers (Ecofin Council), decided on the basis of a Commission proposal that the Slovak koruna will be replaced by the euro at the rate of SKK 30.1260 to the euro. "

From the National Bank of Slovakia:

The [Economic and Financial Affairs Council] today adopted a decision on adoption by Slovakia of the euro as its currency, as from 1 January 2009. The decision will enlarge the euro area to 16 member states, allowing six months for Slovakia to prepare for the changeover. The Council also adopted regulations setting the permanent conversion rate for the Slovak koruna to the euro, and adjusting certain technical provisions on the euro. Euro notes and coins will be issued in Slovakia at the same time as adoption of the euro. The conversion rate is set at 30.1260 Slovak koruna to the euro, which corresponds to the current central rate of the koruna within the EU's ERM II exchange rate mechanism. 

Fifteen of the EU's 27 member states currently use the euro as their currency: Belgium, Cyprus, Germany, Greece, Spain, France, Ireland, Italy, Luxembourg, Malta, the Netherlands, Portugal, Austria, Slovenia and Finland. Euro notes and coins were introduced in 12 of those countries on January 1, 2002, in Slovenia on January 1, 2007 and in Cyprus and Malta on January 1, 2008.

Incremental expansion of the euro zone is reflected in the DataBuffet geo codes IEZ12, IEZ13, and IEZ15.  The code IEUZN refers to an increasing set of countries over time, as explained in this DataBuffet.com Monthly Update article from March 2007:( http://www.economy.com/data/ocb/pro/DataBuffet_Monthly_Update/DataBuffet_Monthly_Update_March_2007.asp#new_geo_eurp ).