Even as mythology this was impossible to stomach.

First you say that they had to "leave Earth as soon as possible," and one sentence later you say they engineered Homo sapiens and created the Mayan calendar so we'd know "when Niribu will next return, after 3600 years."

Uhh... so they had to leave in a hurry because Planet X was moving out of shuttle range or whatever, but they had enough time to sit around through 10,000 years of agricultural human existence, then thousands of years more while civilizations came and went, until finally the Mayans came along and they were able to write down a "Will Return On..." note?

You really need to think through your timeline before you start writing short stories. I would suggest you make an outline next time, before you commit to a rough draft - it helps keep things in a sequential order that contributes to the plot rather than subtracting from it.

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