# Time index

Dynamic simulation time periods are specified in the system variable Time. To perform dynamic simulation, you must provide a definition for **Time**.

To edit the definition of Time, select **Edit Time** from the Definition menu to open the Object window for **Time**.

Time is defined by default as a list of three numbers 0, 1, and 2. You might want to define **Time** as a list of years, as in the following example.

Time becomes the index for the array that results from the Dynamic function.

Tip

A model can have only one definition **Time**— that is, one set of time periods for Dynamic functions. Any number of variables in the model can be defined using Dynamic().

Tip

A variation, Dynamic[T](), can be used to represent recurrences over indexes other than **Time**, but placing the index name in square brackets. This provides a way to express secondary recurrences if you’ve already used your Time index for sometime else. The dynamic concepts are introduced thoroughly in this chapter using

**Time**, but if you have a loop using a different index, just substitute your other index for Time in what follows.

## See Also

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