Excel 2007 on Win 7 Pro

In brief, structure of worksheet is this:
Col B - date (one row per day, every day)
Col S - A number, generally trending toward zero
Col S - function to predict when S will reach zero: =FORECAST(0,B$7:B10,S$7:S10)

Two issues:
First, very rarely there will be an aberration in the value of col S, which pulls the 'date to zero' very far forward - even into the past. I can manually delete these skewed values, but it seems there could be a better way to deal with this than deleting historical data.

Second, I also have a chart with a trendline to visualize 'date to zero'. This trendline is show a zero date much further into the future than the formula in the spreadsheet. I've been relying on the chart for quite some time and just yesterday decided to look for a forumula I could put in the worksheet itself - leading me to the FORECAST function. My reading said the charting trendline uses the FORECAST function, but since I have such a discrepancy it would appear I've got apples and oranges, but don't know where to look or how to reconcile the difference.