How can I convert the dates returned by FinancialData into decimal representation?

Question

I'm trying to run a regression using Fit on the output of FinancialData["SPY", "Jan. 1, 2011"]. However, the dates returned from FinancialData are not in a format that is recognized by Fit. The function returns dates in the format {y,m,d}, and I need the dates to be expressed in decimal form. I'm relatively new to Mathematica, but I've got to believe there's an easy way to transform the list provided by FinancialData into one that has the format of {<date in decimal>, price}. Any guidance would be most appreciated.

Answer 1

data = FinancialData["SPY", "Jan. 1, 2011"] /. {d_List, v_} :> {AbsoluteTime@d, v};
model = a x^4 + b x^3 + c x^2 + d  x + e;
fit = FindFit[data, model, {a, b, c, d, e}, x]
modelf = Function[{x}, Evaluate[model /. fit]]
Plot[modelf[x], {x, Min@data[[All, 1]], Max@data[[All, 1]]}, Epilog -> Map[Point, data]]

Edit

Better (tick labels showing dates)

data = FinancialData["SPY", "Jan. 1, 2011"] /. {d_List, v_} :> {AbsoluteTime@d, v};
model = a x^4 + b x^3 + c x^2 + d  x + e;
fit = FindFit[data, model, {a, b, c, d, e}, x]
modelf = Function[{x}, Evaluate[model /. fit]]

p = Plot[modelf[x], {x, Min@data[[All, 1]], Max@data[[All, 1]]}, 
       Epilog -> Map[Point, data]];
s = AbsoluteOptions[p, Ticks] /. {o_ -> s_List} :> s;
s1 = s /. {v__, w__} :> {(MapAt[
     If[# != "", 
          DateString[DateList[#], {"Month", "/", "YearShort"}], 
          Null] &, 
     #, {2}] & /@ v), w};
Show[p, Ticks -> s1]

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Or, using DateListPlot, with the model sampled at the original dates:

DateListPlot[{data, Table[{d, modelf[d]}, {d, data[[All, 1]]}]}, 
   Joined -> {False, True}]