Forecast question

[Jack]

I'm working on the handbook chapter for Forecasts, and cannot figure  
out how the "Day of month to start forecast" is actually used.  I have  
not yet read the code, but I suppose that will be my next step.

If the cycle is 30 days, and I start with day 0, I get predictions for  
30, 60, and 90 days.  If I set it to 1, I get 11, 41, 71, and 101  
(which is above the 96 Days to Forecast.)  That pattern follows up to  
19 days (29, 59, 89, 119) but at 20, I get 30, 60, and 90, dropping the  
fourth value.  With 21, I get 1, 31, 61, 91, back to four cycles.

Any explanations are welcome.

Jack