Question: What is the birthdate of this King of England, son of Edward, the Black Prince, whose court companion was Robert de Vere, Duke of Ireland?

Context articles: Robert de Vere, 6th Earl of Oxford
Hugh de Vere, 4th Earl of Oxford
Barony of Bannockburn
Robert de Vere, Duke of Ireland
Maud de Ufford, Countess of Oxford
Richard II of England
Horace Vere, 1st Baron Vere of Tilbury
Robert de Vere, 5th Earl of Oxford
Robert de Vere, 3rd Earl of Oxford
Aubrey de Vere, 2nd Earl of Oxford

Robert de Vere, 6th Earl of Oxford (c. 24 June 1257 – 17 April 1331) was the son and heir of Robert de Vere, 5th Earl of Oxford, by his wife Alice de Sanford.
Hugh de Vere, 4th Earl of Oxford (c. 1208 – December 1263) was the only son and heir of Robert de Vere, 3rd Earl of Oxford and Isabel de Bolebec, daughter and eventual sole heiress of Hugh de Bolebec.
The Barony of Bannockburn is a Scottish feudal barony located in the historical county of Stirlingshire.
 The first barons of Bannockburn were from the Sandilands and Vere families, descendants of a man named James who around 1348 had been granted the territory of Sandilands, Lesmahagow Lanarkshire, taking the territorial name as his surname.
 He was also later granted the barony of Calder near Edinburgh.
 The link of the Sandilands to Bannockburn was established on 14 July 1489, when King James IV confirmed James Sandelands of Calder and his wife Margaret Kerr his wife in the lands of Bannockburn.
 By 7 July 1509, James Calder had transferred Bannockburn to his son.
 On that date, King James IV confirmed a charter by Sir James Sandelands of Calder granting various lands including the barony of Ouchterbannok alias Bannockburn to his son James Sandelands.
 On 23 August 1510, King James IV granted Sir James Sandelands of Caldour (nephew of Sir James Sandelands and his wife Mariota Forrester), various lands including the lands and barony of Bannockburn.
 Robert De Vere the 6th.
 Earl of Oxford accompanied Edward the 2nd.
 on his campaign which culminated in defeat by the victorious King Robert the Bruce at the Battle of Bannockburn on the 23rd and 24th.
 of June 1314.
 It is by a quirk of fate that the barony of Bannockburn was granted to the Sandilands as they were close relatives to the senior Scottish branch of the Veres both in blood and territorial location their estates in Lanarkshire being contiguous.
Robert de Vere, Duke of Ireland, Marquess of Dublin, and 9th Earl of Oxford KG (16 January 1362 – 22 November 1392) was a favourite and court companion of King Richard II of England.
 He was the ninth Earl of Oxford and the first and only Duke of Ireland and Marquess of Dublin.
Maud de Ufford, Countess of Oxford (1345/1346 – 25 January 1413) was a wealthy English noblewoman and the wife of Thomas de Vere, 8th Earl of Oxford.
 Her only child was Robert de Vere, 9th Earl of Oxford, the favourite of King Richard II of England.
 In 1404 in Essex, she took part in a conspiracy against King Henry IV of England and was sent to the Tower of London; however, she was eventually pardoned through the efforts of Queen consort Joanna of Navarre.
Richard II (6 January 1367 – c. 14 February 1400), also known as Richard of Bordeaux, was King of England from 1377 until he was deposed on 30 September 1399.
 Richard, a son of Edward, the Black Prince, was born in Bordeaux during the reign of his grandfather, Edward III.
 His father was Prince of Aquitaine.
 Richard was the younger brother of Edward of Angoulême, upon whose death Richard, at three years of age, became second in line to the throne after his father.
 Upon the death of Richard's father prior to the death of Edward III, Richard, by primogeniture, became the heir apparent to the throne.
 With Edward III's death the following year, Richard succeeded to the throne at the age of ten.
Horace Vere, 1st Baron Vere of Tilbury (1565 – 2 May 1635) (also "Horatio Vere" or "Horatio de Vere") was an English military leader during the Eighty Years' War and the Thirty Years' War, a son of Geoffrey Vere and brother of Francis Vere.
 He was sent to the Palatinate by James I in 1620.
 He was created Baron Vere of Tilbury, and died without a male heir.
Robert de Vere, 5th Earl of Oxford (c. 1240 – 1296) was the son and heir of Hugh de Vere, 4th Earl of Oxford.
Robert de Vere (after c. 1165 – before 25 October 1221), hereditary Master Chamberlain of England, was son of Aubrey de Vere, 1st Earl of Oxford, and Agnes of Essex.
 He succeeded his brother as the third Earl of Oxford, and was one of the twenty-five guarantors of Magna Carta.
Aubrey de Vere, 2nd Earl of Oxford (c. 1163 – 1214), hereditary Master Chamberlain of England, served in military campaigns under King Richard and King John.
 He was succeeded in the earldom by his brother, Robert de Vere, 3rd Earl of Oxford.