First, using Chart Trendline, if you go to the Options tab, you can
select Display Equation and Display R-Squared. The latter provides
some indication of closeness of fit; the closer to 1 the better.
Second, in a cell, you can use the SLOPE() and INTERCEPT() functions.
Since date serial numbers are in days, the unit of the slope is days.
You can compute any point on the trendline by:
=B7*slope(B1:B6,A1:A6) + intercept(B1:B6,A1:A6)
where B7 contains a date.