What are the units of x in a date-based trend equation? If I take a
simple integer-integer dataset,

1 1
2 4
3 9
4 16

and fit a second-order polynomial to it, I get y = 1x^2 +
5.6843418860808e-14, which is reasonably accurate at the known points.
If, however, I have dates in my x axis, I can't figure out the units
of the equation.

1/1/2001 1
1/2/2001 4
1/3/2001 9
1/4/2001 16
1/5/2001 25
1/6/2001 36
1/7/2001 49
1/8/2001 64
1/9/2001 81
1/10/2001 100

yields y = x^2 + 7.3782x + 1.360945881, but what values for x yield
the y's I already know? It's not the integer of the date--that series
starts 1.361292e+09, 1.361366e+09, 1.361439e+09.