This question is all kinds of wrong. If you take the literal meaning of the question, then the answer is 1987 - when Billy Joe and Mike formed the band "Sweet Children". If you take the question to mean "when did they start performing as Green Day?", the answer would be 1989, when they dropped the Sweet Children moniker in order to not be confused with another local band, Sweet Baby. While I do see that 1988 is in the middle of these two, it still isn't a legitimate answer.
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stupid portugese