It is often useful to be able to use current simulation time in your model: for example, you may want to incorporate a gradual increase in some external variable such as temperature. For this purpose, a special function called *time* is provided in Simile.

The *time* function has a value equal to the number of time units that has elapsed since the start of the simulation (which is assigned a time of zero). To help avoid any confusion, please note that it has nothing to do with the real-world time that elapses as the model runs. How long a simulation takes to run depends on how complex it is and how fast your computer is, amongst other factors, but none of these influences the *time* function. If you run a simulation for 100 time units, then the *time* function will increase from zero to 100 during the simulation. This may take 20 seconds of your time or it may take 20 minutes, depending as I say, on the model’s complexity and the speed of your computer.

You can use the *time* function in constructing expressions in Simile’s equation language. This gives enormous flexibility in constructing expressions. Perhaps the two most common uses are in drawing sketch graphs to illustrate how some external influence changes through time, and in constructing cycles, such as the repetition of the seasons year after year.

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