Question 1160043
95% CI  half-interval  is t(0.975, df=87)*15.7/sqrt(88)
t is 1.99
CI half-interval is 3.33
(1.57, 8.23) units days

We don't know what the exact mean is, and at any given point in time there is one. But we are 95% confident that the true mean lies in the interval. Stated another way, if we constructed 100 such intervals with the same sample size, we would expect the true mean to lie within 95 of them, but we wouldn't know which 95.