Context: Some hint of Cnut's childhood can be found in the Flateyjarbók, a 13th-century source that says he was taught his soldiery by the chieftain Thorkell the Tall, brother to Sigurd, Jarl of mythical Jomsborg, and the legendary Joms, at their Viking stronghold on the island of Wollin, off the coast of Pomerania. His date of birth, like his mother's name, is unknown. Contemporary works such as the Chronicon and the Encomium Emmae, do not mention this. Even so, in a Knútsdrápa by the skald Óttarr svarti, there is a statement that Cnut was "of no great age" when he first went to war. It also mentions a battle identifiable with Sweyn Forkbeard's invasion of England and attack on the city of Norwich, in 1003/04, after the St. Brice's Day massacre of Danes by the English, in 1002. If Cnut indeed accompanied this expedition, his birthdate may be near 990, or even 980. If not, and if the skald's poetic verse references another assault, such as Forkbeard's conquest of England in 1013/14, it may even suggest a birth date nearer 1000. There is a passage of the Encomiast  with a reference to the force Cnut led in his English conquest of 1015/16. Here  it says all the Vikings were of "mature age" under Cnut "the king". A description of Cnut appears in the 13th-century Knýtlinga saga:

Question: What besides his birthday is unknown about Cnut?

Answer:
his mother's name