The below is derived from dialogue given in the various episodes of the program. Currently it only covers the history of the island prior to the arrival of the castaways on the island. Eventually, I intend to cover the events of the series itself.
ca. 30,000 B.C.: People of unknown origins visit the island, leaving behind behind a stone tablet engraved with instructions on how to get to the island. (1)
ca. 300 B.C.- A.D. 500: Polynesian explorers first reach the island.(2) Different groups, including the Marubi(3), the Matoba(4), the Kapuans(5) and the Kupaki(6), will visit the island over the years to come.
ca. A.D. 500-900: People who have had contact with the Mayan civilization of Central America visit the island. (7)
ca. A.D. 1666: The "mind-reader" plant is believed to have become extinct.(8)
ca. Approximately A.D. 1713-1763: An ancestor of Jonas "The Skipper" Grumby acquires a ring which will still be in his family in 1964.(9)
ca. A.D. 1780-1880: A chest full of cannon balls is buried on the island.(10)
A.D. 1906: Thieves rob the San Francisco Art Museum, then hide out on the island. When they leave the island, they leave behind a parrot named "Sam".(11)
ca. A.D. 1917-1918: "Wrongway" Feldman serves in World War One. (12) Gilligan's Uncle Ramsey serves as a guide in the same war. (13) An attache case containing classified U.S. plans for defense against enemy attack are lost at sea in the Pacific Ocean.(14)
ca. A.D. 1917-1927: A yacht belonging to Fifi Lafrance and Ricardo Laughingwell sinks in the lagoon of the island.(15)
A.D. 1929: The Howell family loses alot of money in the stock market crash.(16)
A.D. 1931: "Wrongway" Feldman crashes on the island while on an around-the-world flight. (12)
ca. A.D. 1942-1945: Japanese troops occupy the island during World War Two, but are eventually driven off the island by U.S. forces.(17)
A.D. 1942: Jonas "The Skipper" Grumby serves aboard a sub-chaser during the Battle of Guadalcanal.(18) The Japanese submariner that visited Gilligan's Island claims that his radio broke in this year.(19)
A.D. 1944: Thurston Howell III and Lovey Wentworth are married(20). The service is performed by the Reverend Norris Buckley. (21) Consolidated General jumps 17 points.(22)
A.D. 1947: Jonas "The Skipper" Grumby is in a bar fight in Singapore. (23)
A.D. 1955: The painter Alexandri Dubov begins his voluntary exile on the island. (24)
ca. A.D. 1956: The "Jungle Boy" is marooned on the island. (25)
ca. Approximately A.D. 1960-1963: The Mosquitoes hit it big.(26)
A.D. 1963: Gilligan buys a ticket in the Argentinean Sweepstakes.(27)
A.D. 1964: Randolph Blake is killed. (28) The passengers and crew of the S.S. Minnow are shipwrecked on the island.(29)
(1)In "The Secret of Gilligan's Island", the Professor says the tablet may be a million years old. This strikes me as extremely unlikely. From what I've read, the earliest form of pre-literate "symbolic" systems were created around 30,000 B.C., so I placed the tablet's creation in this timeframe.
(2)This is the timeframe in which the original Polynesian settlers are said to have reached Hawaii, so I'm assuming the island would've been first visited by the Polynesians around the same time.
(3)Mentioned in "Two on a Raft".
(4)Mentioned in "Slave Girl".
(5)Mentioned in "Gilligan, the Goddess".
(6)Mentioned in "High Man on the Totem Pole"
(7)The Professor says that the relics Gilligan finds in "Voodoo" show a Mayan influence. I chose this timeframe as it represents the high point of the Mayan civilization, and thus to my mind the time when they might have been most likely to influence the island peoples.
(8)The Professor mentions in "Seer Gilligan" that the mind-reader plant was believed to have become extinct three hundred years before.
(9)The Skipper says in "Waiting for Watubi" that his ring has been in his family for over two hundred years, so I would think it would fall roughly into this timeframe.
(10)The cannon balls are found in "Plant You Now, Dig You Later". To my knowledge, none of the native peoples in the area used cannons, Europeans didn't reach this area until around 1780, and by 1880, cannon balls had been replaced by more modern weapons, so this is the timeframe I chose. Someone who knows more about the history of weapons than I do could probably pick a better range.
(11)Described in "New Neighbor Sam".
(12)Mentioned in "Wrongway Feldman".
(13)Mentioned in "How to be a Hero".
(14)Mentioned in "The Invasion"
(15)Mentioned in "Castaways Pictures Presents". No date was given, but I chose this range as the most likely, since it was the heyday of silent film.
(16)Mentioned in "Will the Real Mr. Howell Please Stand Up?".
(17)American ammunition from the war was found on the island in "Forward March", and the Professor speculates that the gorilla observed humans in combat. A Japanese ammunition pit was found in "And Then There Were None". I am making the assumption based on the above that a battle in the Pacific War was fought on the island.
(18)Mentioned in "Goodnight Sweet Skipper".
(19)Mentioned in "So Sorry, My Island Now".
(20)Mentioned in "Home Sweet Hut".
(21)Mentioned in "Mr & Mrs. ???".
(22)Mentioned in "Angel on the Island".
(23)Mentioned in "Waiting for Watubi".
(24)Mentioned in "Goodbye Old Paint".
(25)In "Gilligan Meets Jungle Boy", the Jungle Boy appears to be about 14 years old. Since he can't speak, I'm positng that he's been on the island since he was about five, and lost the power of speech due to lack of human contact. As to how he survived? Maybe one of the island's apes raised him up.
(26)In "Don't Bug the Mosquitoes", Gilligan says that he has all their albums, which implies to me that they had to have been around at least a few years before the shipwreck.
(27)In "The Sweepstakes", the Howells note that Gilligan's ticket is two years old.
(28)Mentioned in "Not Guilty".
(29)Proceeding rom the broadcast date of "Two on a Raft".