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I belive I know the general year of the Death of Master Chief, it is explained below.

According to records the battle of New Mombasa took place on 2552, Testimonial: Gravesite takes place 43 years after, and he said that the "ceremony" were his grave is was 5 years ago, though he is not proclaimed dead but MIA.

The Math:

2552
+ 43
____
2595
-5
____
2590

That is not the year of the end of Halo 3, when he goes MIA, this is the year

Those Believe ads were NOT canon. They had as much information about the game's ending prior to its release as we did.

they were pretty nice though