Passport Validity Requirements
Entry into Greece or any of the other European countries in the Schengen area for short-term tourism, a business trip, or in transit to a non-Schengen destination requires that the passport be valid for at least three months beyond the intended date of departure. If the passport does not meet the Schengen requirements, the traveler may be refused boarding by the airline at the point of origin or while transferring planes. A traveler could also be denied entry upon arrival in the Schengen area.
For example, some countries require all travelers have at least six months of validity on their passports, even for transit purposes. Additionally, some airlines or countries may assume a U.S. citizen intends to enter the EU and stay the maximum of 90 days. They will want to see another three months of validity beyond that date. If you have a return ticket showing your departure is less than 90 days in the future, that might be accepted. However, for this reason, we recommend that U.S. passports have at least six months validity remaining whenever traveling abroad to avoid potential travel disruptions.
The Schengen Countries, as of September 2014, are: Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and Liechtenstein.