This date shifting is a result of the discrepancy between a human calendar year—which is usually counted as 365 days—and an astronomicalyear, which is about 365.24 days.
这个日期的变化是人类的历年（通常以365天为计）与天文年（以365.24天）之间的差异而产生的。
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It turns out, however, that there are 365.24219 days in an astronomical "tropical" year-defined as the time it takes the sun, as seen from Earth, to make one complete circuit of the sky.
So Emmerich found a way to kill the Earth using a faux astronomical event (there is no planetary alignment in the year 2012) and by inventing a solar interaction that has never been observed.