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This Python program runs in 80ms.

Run:[ @repl.it ]Solution:Grandad would have celebrated his 107th birthday in 1999.Grandad was born in 1892.

There are 43 possible dates:

If we assume “summer” limits the months to June, July, August (northern hemisphere), then these possibilities are narrowed down to 13 possible dates.

If we assume “summer” limits the months to December, January, February (southern hemisphere), then these possibilities are narrowed down to the 4 possible dates in December. So Grandad could have been born on Christmas Day in Australia.

But all possible dates are in 1892, so it is not really necessary to narrow these down further.

His 6th birthday was in 1898, his 12th in 1904, his 18th in 1910, and 1994 was his 102nd birthday.

1900 was not a leap year, of course.

His birthdays fell on a Sunday in 1898, 1904, 1910, 1921, 1927, 1932, 1938, 1949, 1955, 1960, 1966, 1977, 1983, 1988. Somehow I suspect that was the last: there would be no need to assume he lived to 102.