I've solved the problem I've been having with setting the intervals on Excel's histograms. I tried adjusting the bounds by setting the Overflow and Underflow bins to ±18.0 but they were ignored. Apparently, these bounds need to be less than the maximum and minimum for the data series used for the histogram. It's debatable whether or not one needs empty columns at the ends of a histogram. These entries set up the histogram in an acceptable manner.
Again, the observed counts from the histogram can be used for comparison with the statistically expected counts for a normal distribution.
The distribution above looks similar to a normal distribution and the z-scores for the intervals can't be rejected on statistical grounds but one cannot always get a clear impression of the actual distribution for deviations from a mean value with a single dataset. In another histogram the distribution appears to be more like a trapazoid.
We would need more data to come to a conclusion about what the actual distribution for the Equinox times is.