There was a thread on this last year. The top answer (also my favorite), by /u/by_whom, was:

I would start with the end of World War II. Geologists already define the "present" as any time after the first atomic bomb was detonated, as these bombs increased the amount of carbon in the atmosphere to the point where traditional carbon dating methods will not work after this year. This provides a clean divide with the change in carbon dating.

The post atomic world is also one in which humanity has changed in so many ways, that I think it's compelling enough to start the years there.

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