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Is there a way to specify a window-length = as far as the data goes back?

I'd like to define a CustomFactor that looks at the prices for all available dates before the current timestamp and tries to compute some output. Is there a way to do this? It seems like I can only specific a static value for window_length when I define my CustomFactor. Can I get around it by just specifying a very high window length, like 10000?

Ex:

Time              Price  
2002/1/1     45.4  
2002/1/2     42.4  
2002/1/3     41.4  
2002/1/4     41.4  
...
2011/1/1     125.5  

At 2002/1/1, I'd like the window_length = 0
At 2002/1/2, I'd like the window_length = 1
At 2002/1/3, I'd like the window_length = 2
At 2002/1/4, I'd like the window_length = 3
so on and so forth. At 2011/1/1, I'd like the window_length = I guess 365 * 10 years.