Swallowdale has an internal date of 1931 (the note they leave in the cairn on the summit of the Kanchenjunga) and all the books fit in a four year period.

True, but Swallows and Amazons also has an internal date of 1929 (the treaty signed when the Ss & As first meet on the island) - which makes absolutely no sense at all as all the evidence indicates the two books take place one year apart!