# What is DT?

Many have asked, few have understood. DT, or dt (depending on your level of disdain), is the interval of time between calculations. DT is expressed in whatever time unit you've chosen for your model. Therefore, DT answers the question: Is my model having its numerical values re-calculated once every time period, twice, three times...?

If you do not make a conscious choice of time unit for your model, the default choice will be "Months" (ithink) or "Time" (STELLA). To see this, visit the Run Specs... dialog located under the Run menu. Your choice of time unit provides the denominator of the units-of-measure for all of the flows in your model. For example, if you have flows of widgets, people, and dollars (and you are using the default time unit of "Months"), then the units-of-measure for your flows will be widgets/month, people/month, and $/month. If DT in this model is 1.0, then a round of calculations will be performed once each month. If DT is 0.25, then a round of calculations would be performed every 1/4 of a month (or, four rounds of calculations would be performed per month). And, so on.