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Model Execution 607
Time units
global time unit for a discrete rate model. If Calendar dates has been selected, individual Rate library blocks will not be able to select Months or Years as their local time unit.
To enable Calendar dates:
Choose Run > Simulation Setup > Setup tab Select Use Calendar dates
Click the Start: field
A calendar appears to select a date and time that represents the beginning of the simulation. ExtendSim will use that information and the other entries you have made in the Setup tab to calculate the End date. (Of course, all dates are in simulation time, not in real time.)
There are two other options that affect Calendar date definitions. The European format option places the day before the month. The Macintosh date option, selected by default for Mac OS, is needed when Excel is set to use the “1904 date system” (see Excel > File > Prefer- ences > Calculation.) If “1904 date system” is not checked in Excel on the Macintosh, uncheck this option in ExtendSim.
Selecting Use calendar dates for a model can also affect blocks that deal with time. For example, if Calendar dates is enabled, you can use the calendar format to create a schedule in the Create block (Item library). An example of this is the block named “Schedule” in the Scheduling Resources model located in the folder \Examples\Discrete Event\Resources and Shifts. (The model is discussed in the section “Scheduling resources” on page 230.)
The following blocks allow you to select Calendar dates in their dialog:
• Create (Item library) when it is set to Create items by schedule or Create values by
schedule
• History and Shift blocks (Item library)
• LookupTable(Valuelibrary)whenitissettoLookupthe:time
• Plotter blocks such as the Plotter, Discrete Event and Plotter I/O
Do not use Calendar dates if block dialogs are set to use values smaller than 1 second (such as 0.0005 seconds), as it may affect numerical precision.
Time unit conversions (non-Calendar dates)
If a model has not been set to use Calendar date definitions when it runs, ExtendSim converts all time-based parameters from their local time units to the specific global time unit based upon constants entered in the Run > Simulation Setup > Setup tab. You can use the default settings or set your own definitions for all the time categories. Specifying that there are 8 hours in a day is an easy way to model a standard 8 hour working day. Note that the default value for a year is 360, not 365.
Default constants in the Simulation Setup dialog
For example, assume you are using the default constants and the global time unit for the model is days, but you have selected a local time unit of hours for a block’s time-based parameter. When the model runs, ExtendSim will cause that parameter to be divided by 24, converting it into the appropriate value for one day.
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