code: @SuppressWarnings("checkstyle:magicnumber")
      private static double phi(long timeDiffMillis, double meanMillis, double stdDeviationMillis) {
          double y = (timeDiffMillis - meanMillis) / stdDeviationMillis;
          double e = Math.exp(-y * (1.5976 + 0.070566 * y * y));
          if (timeDiffMillis > meanMillis) {
              return -Math.log10(e / (1.0 + e));
          } else {
              return -Math.log10(1.0 - 1.0 / (1.0 + e));
          }
      }
docs: Calculation of phi, derived from the Cumulative distribution function for
  N(mean, stdDeviation) normal distribution, given by
  1.0 / (1.0 + math.exp(-y * (1.5976 + 0.070566 * y * y)))
  where y = (x - mean) / standard_deviation
  This is an approximation defined in β Mathematics Handbook (Logistic approximation).
  Error is 0.00014 at +- 3.16
  The calculated value is equivalent to -log10(1 - CDF(y))