The evolution of life on Earth - a tale of both slow and abrupt changes over time - emphasizes that change is pervasive and ever present. Change affects all disciplines using observational data, especially time series of observations. When the dates of events matter, so data are not ahistorical, they are called non-stationarity denoting that some key properties like their means and variances change over time. There are several sources of non-stationarity and they have different implications for modelling and forecasting.
Castle, J.L. and Hendry, D.F. (2019). Modelling our Changing World, Palgrave Macmillan. Open Access.