# Lesson 1 - Unit 10 Overview

"The only reason for time is so that everything doesn't happen at once."

- Albert Einstein

As you may recall from Unit 6, there are two ways to carry out a dynamic simulation in GoldSim (specified by selecting the Time Basis in the Simulation Settings dialog):

• In an “Elapsed Time” simulation (the default and what we have been using so far), you specify a simulation Duration.  The simulation is then tracked in terms of the elapsed time since the simulation began.  When plotting a time history result, the x-axis would be labeled in terms of elapsed time (i.e., it would start at zero).
• In a "Calendar Time" simulation, you enter a Start Time and an End Time, and the simulation is tracked in terms of the calendar date/time (i.e., GoldSim tracks things like what hour of the day, day of the week and month it is during the simulation). When plotting a time history result, the x-axis would be labeled in terms of elapsed year, month, day or hour (i.e., it would start at the Start Time).

All of the dynamic simulation models that we have explored and built so far have been “Elapsed Time” models.

If the simulations you are running are very short (e.g., minutes or hours) or very long (e.g., hundreds of years), in most cases an Elapsed Time simulation would be appropriate.  However, when your simulation time is between these two extremes, it is quite possible that you will want to run a Calendar Time simulation.  This is because some inputs, or the behavior of the system itself, might depend on the time of day or the date, and hence you will want to specifically track and reference this information in your model.

Here are some examples of models where the time of day or date would be important:

• You are simulating a facility or operation that is only operated during certain hours of the day and/or days of the week.
• You are simulating a system that has important diurnal variations (e.g., sunlight, temperature).
• You are simulating a system that has important seasonal variations (e.g., rainfall).

This Unit will describe how to create models in which you specifically track and reference calendar dates or time of day. In particular, in this Unit we will discuss the following:

• Creating calendar-based models;
• Referencing dates and times using Run Properties;
• Modeling seasonal variables in a Lookup Table;
• Defining Time Series;
• Using historic data in a Time Series element;
• Modeling seasonal variables in a Time Series element;
• Using Reporting Periods; and
• Computing moving averages using Integrator elements.

Some of these features (Time Series, Reporting Periods, moving averages) are not limited to calendar-based models.  However, they are most typically used for such models.

This Unit includes three Exercises, as well as several Examples that we will work through together.

This Unit has a total of 11 Lessons (including this overview and a summary at the end).