What is the validity duration of FAA Medical for international flights? FAA Rule:

The duration of FAA medical certificates is specified in 14 CFR 61.23.

An example would be for:


A first-class medical certificate
Applicant is under age 40
Applicant is an airline transport pilot certificate for pilot-in-command privileges


In this case:


  Then your medical certificate expires, for that operation, at the end of the last day of the 12th month after the month of the date of examination shown on the medical certificate.


So this clearly states that if I pass my medical on January 1st 2014 then my medical is good until the end of the day on January 31st 2015.

ICAO Rule:

However, I have been told that ICAO rules state that the medical expires 12 months to the day from when it was issued, which would mean that it would no longer be valid as of January 1st, 2015.  (A reference for this would be great.)

Question:

As the holder of an FAA pilot license, with the above FAA issued medical, may I legally fly outside of the US after January 1st 2015 (but before February 1st 2015)?
I believe the ICAO regulation you are thinking of is this one:


  Excerpts from ICAO Annex 1- Personnel Licensing
  
  “1.2.5.2 Except as provided in 1.2.5.2.1, 1.2.5.2.2, 1.2.5.2.3,
  1.2.5.2.4 and 1.2.5.2.5, a Medical Assessment issued in accordance with 1.2.4.5 and 1.2.4.6 shall be valid from the date of the medical
  examination for a period not greater than:
  
  60 months for the private pilot licence — aeroplane;  12 months for
  the commercial pilot licence — aeroplane;  12 months for the airline
  transport pilot licence — aeroplane;”
  
  It also indicates the following in a note:
  
  “Note 2 — When calculated in accordance with 1.2.5.2 and its
  sub-paragraphs, the period of validity will, for the last month
  counted, include the day that has the same calendar number as the date
  of the medical examination or, if that month has no day with that
  number, the last day of that month.”
  
  In ICAO Annex 1, 2.4.1.6, the medical requirement for a commercial
  pilot is prescribed. It states:
  
  “2.4.1.6 Medical fitness
  
  The applicant shall hold a current Class 1 Medical Assessment.”
  
  In ICAO Annex 1, 2.5.1.6, the medical requirement for an airline
  transport pilot is prescribed. It states:
  
  “2.5.1.6 Medical fitness
  
  The applicant shall hold a current Class 1 Medical Assessment.”
  
  “6.1.1 Classes of Medical Assessment
  
  Three classes of Medical Assessment shall be established as follows:
  
  a) Class 1 Medical Assessment; applies to applicants for, and holders
  of:  — commercial pilot licences — aeroplane and helicopter  — airline
  transport pilot licences — aeroplane and helicopter  — flight
  navigator licences  — flight engineer licences
  
  b) Class 2 Medical Assessment; applies to applicants for, and holders
  of:  — private pilot licences — aeroplane and helicopter  — glider
  pilot licences  — free balloon pilot licences
  
  c) Class 3 Medical Assessment; applies to applicants for, and holders
  of:  — air traffic controller licences.”


However, ICAO is not involved in enforcement but rather coming up with standards, which would then be enforced (if adopted) by the member country's aviation authority.  

From the FAA:


  There are many differences to be aware of when flying outside the
  United States (U.S.). International Civil Aviation Organization (ICAO)
  rules prevail. Member states follow ICAO guidelines as published, with
  differences noted, in their official Aeronautical Information
  Publication (AIP).
  http://www.faa.gov/air_traffic/publications/ifim/intl_overview/


For example, the EASA (European Aviation Safety Agency) has rules very similar to the FAA:


  MED.A.055  Validity, revalidation and renewal of medical certificates 
  (a)  Validity  (1) 
  Class 1 medical certificates shall be valid for a period of 12 months. 
  (2) 
  Notwithstanding to paragraph (1) the period of validity shall be reduced to 6 months for pilots 
  who:  (i) 
  are engaged in single­pilot commercial air transport operations carrying passengers and 
  have reached the age of 40  (ii)  have reached the age of 60
  (http://easa.europa.eu/rulemaking/docs/npa/2008/2008-17/NPA%202008-17c.pdf)


However, according to this report: http://dspace.mit.edu/bitstream/handle/1721.1/35725/FlightOpsICATfinal2.pdf,


  Air carrier operations are generally regulated by the country of
  registration and the sovereignty in which the operation is conducted,
  including the Federal Aviation Administration (FAA) in the USA and the
  Joint Aviation Authority (JAA) in Europe. International flights may
  also fall under the jurisdiction of ICAO (International Civil Aviation
  Organization) when operating abroad (U.S.-based carriers are governed
  by both FAA  and ICAO regulations when conducting international
  operations.)


So you might be legal until you hit the international waters.  At the very most you might be able to go abroad, but then not be legal to fly back.

The National Business Aviation Association (NBAA) recommends that pilots comply with the strictest interpretation of the law (so in this case, have the appropriate compliance with ICAO standards) to avoid having to explain the nuances between the two sets of regulations. 
(http://www.nbaa.org/admin/personnel/medical/medical-certificates.php)