Question: Prince Axel of Denmark was a cousin of the King of Greece, who ruled during what years?

Context paragraphs:
Prince Nicholas of Greece and Denmark: Prince Nicholas of Greece and Denmark (, 22 January 1872 – 8 February 1938), of the Glücksburg branch of the House of Oldenburg, was the fourth child and third son of King George I of Greece, and of Queen Olga.  He was known as "Greek Nicky" in the family to distinguish him from his paternal first cousin Emperor Nicholas II of Russia.  Prince Nicholas was a talented painter, often signing his works as "Nicolas Leprince."

Prince Axel of Denmark: Prince Axel Christian Georg of Denmark, {'1': ", '2': ", '3': 'RE, SKmd', '4': "} (Danish: "Prins Axel Christian Georg til Danmark"; 12 August 1888 in Copenhagen – 14 July 1964 in Copenhagen) was a Danish prince and a grandson of Christian IX of Denmark On his father's side, he was a first cousin of Christian X of Denmark, Haakon VII of Norway, Constantine I of Greece, George V of the United Kingdom, Nicholas II of Russia, Maud of Wales and Ernest Augustus III, Duke of Brunswick and on his mother's side of Henri, Count of Paris (1908–1999), Orleanist pretender to the French throne.  Prince Axel was a popular patron of sports.  He was a prominent International Olympic Committee member and activist and also a business executive.  In 1963, Prince Axel became the first honorary member of the IOC in history.  He was an officer in the Royal Danish Navy.

Grand Duchess Alexandra Georgievna of Russia: Grand Duchess Alexandra Georgievna of Russia (Russian: Алекса́ндра Гео́ргиевна ); "née" Princess Alexandra of Greece and Denmark (Greek: Πριγκίπισσα Αλεξάνδρα της Ελλάδας και της Δανίας ); 30 August 1870 – 24 September 1891) was the third child and firstborn daughter of King George I and Queen Olga of Greece, who herself was a daughter of a Russian grand duke, and was also a grandchild of Denmark's King Christian IX and Queen Louise.  She was a sister to Constantine I of Greece, and thus aunt of three kings and two queens, Constantine's three sons, who all became kings of Greece, and two of his daughters, who were queens, in name, of Romania and Croatia, respectively.  She was also first cousin of Tsar Nicholas II of Russia, King George V of the United Kingdom, and both King Haakon VII and Queen Maud of Norway, as well as a paternal aunt of Prince Philip, Duke of Edinburgh.

Greek royal family: The Greek royal family (Greek: Ελληνική Βασιλική Οικογένεια) is a branch of the House of Glücksburg that reigned in Greece from 1863 to 1924 and again from 1935 to 1973.  Its first monarch was George I, the second son of King Christian IX of Denmark.  He and his successors styled themselves "Kings of the Hellenes".  Most members of the dynasty (aside from the last king to reign, Constantine II and his queen consort, Anne-Marie of Denmark) hold the title "Prince or Princess of Greece and Denmark" with the style "Royal Highness", except Marina, Consort of Prince Michael, and their daughters Princess Alexandra and Princess Olga.

Guldborgsund Bridge: The Guldborgsund bridge (Danish: "Guldborgbroen" ) spans the northern end of the Guldborgsund, between the islands of Lolland and Falster in Denmark.  The bridge consists of two steel arched spans with a 30m central opening section having two rising bascules.  It was built between 1933 and 1934, is 180m long and 7m wide, carrying two road carriageways and was officially opened by Prince Axel 6 October 1934.

Kings of the Hellenes family tree: The following is a family tree for the Kings of the Hellenes of the House of Schleswig-Holstein-Sonderburg-Glücksburg, which ruled Greece between the election of Prince Wilhelm of Denmark (George I) to replace Otto of Greece in 1863 until the declaration of the Second Hellenic Republic in 1924, and again from 1935 until the abolition of the monarchy during the reign of King Constantine II in 1973.  Those who ruled Greece are given in bold capitals.

The Royal House of Boureh Gnilane Joof: The Royal House of Boureh Gnilane Joof (variation : Mbin Boureh Gnilane in Serer) was a royal house founded in the 14th century by Jaraff Boureh Gnilane Joof (var : "Bouré Gnilane Diouf" or "Buré Ñilaan").  He was a member the Serer tribe, from the pre-colonial Kingdom of Sine now part of independent Senegal.  It was the first royal house founded by the Joof family during the Guelowar period (1350 - 1969).  Boureh Gnilane Joof was a royal prince and a Jaraff (var : "Diaraf"), a with the powers of a Prime Minister.  He was neither a Maad a Sinig (king of Sine) nor a Maad Saloum (king of Saloum) but a royal prince who had the title "Jaraff" bestowed upon him by his cousin and brother-in-law - Maad a Sinig Diessanou Faye (king of Sine).  His father Maad Patar Kholleh Joof (the conqueror) was the king of Laa and Teigne of Baol (king of Baol).  Boureh's brothers were the first from this house to have succeeded to the throne of Sine during the Guelowar period.  His name was adopted in his honour to refer to the first royal house founded by the Joof family during this dynastic period.  The Joof family of Sine, from this royal house also ruled in the Kingdom of Saloum (the Joof paternal dynasty of Sine and Saloum) The Joof family also ruled in Baol (the Joof paternal dynasty of Baol).  From the date of its foundation up to the abolition of the Serer monarchies of Sine and Saloum in 1969, at least ten kings from this house had succeeded to the throne of Sine.  As the first royal house of Sine founded by the Joof family in this dynastic period, the Royal House of Boureh Gnilane Joof holds great significance in Senegambian, Joof family and , because all the subsequent royal houses founded by the Joof family (who ruled in three Senegambian kingdoms) branched out from this royal house.

Constantine I of Greece: Constantine I (Greek: Κωνσταντῖνος Α΄ , "Konstantínos I"; 2 August [O.S. 21 July] 1868 – 11 January 1923) was King of Greece from 1913 to 1917 and from 1920 to 1922.  He was commander-in-chief of the Hellenic Army during the unsuccessful Greco-Turkish War of 1897 and led the Greek forces during the successful Balkan Wars of 1912–1913, in which Greece expanded to include Thessaloniki, doubling in area and population.  He succeeded to the throne of Greece on 18 March 1913, following his father's assassination.

House of Oldenburg: The House of Oldenburg is a European dynasty of North German origin.  It is one of Europe's most influential royal houses, with branches that rule or have ruled in Denmark, Iceland, Greece, Norway, Russia, Sweden, Schleswig, Holstein, and Oldenburg.  The current Queen of Denmark and King of Norway, the former King of Greece, the consorts of Greece and the United Kingdom, as well as the first twelve names in the line of succession to the British throne, are all patrilineal members of the Glücksburg branch of this house.

Princess Friederike of Hanover: Princess Friederike of Hanover ("German: Friederike Elisabeth Viktoria-Luise Alice Olga Theodora Helena, Prinzessin von Hannover, Prinzessin von Großbritannien und Irland, Herzogin zu Braunschweig-Lüneburg"), (born 15 October 1954 at Salem, Baden-Württemberg, Germany) is the youngest child and only daughter of Prince George William of Hanover and his wife Princess Sophie of Greece and Denmark, sister of Prince Philip, Duke of Edinburgh.  Queen Sofia of Spain and Constantine II of Greece are paternal first cousins and maternal first cousins once removed of Friederike, while she is a first cousin, through her mother, to Charles, Prince of Wales and a goddaughter of Elizabeth II of the United Kingdom.

Supporting paragraphs: Prince Axel of Denmark Constantine I of Greece
Answer: 1913 to 1917 and from 1920 to 1922