TD_ACF calculates the autocorrelation or autocovariance of a time series. Autocorrelation and autocovariance show how the time series correlates or covaries with itself when delayed by a lag in time or space. When TD_ACF is computed, a coefficient corresponding to a particular lag is affected by all the previous lags. For example, the coefficient for lag 4 includes effects of activity at lags 3, 2, and 1.