TIME variable;
The TIME variable is a numeric variable that provides the chronological order or time values for measurements in a DATA= , HISTORY= , or TABLE= input data set. The values of the TIME variable are displayed on the horizontal axis of control charts. If no TIME statement is specified, the observation numbers from the input data set are displayed on the horizontal axis.
When the input data set contains more than one observation with the same TIME value, you must identify the sequences of points to be plotted on the control charts by specifying a SERIES variable.