The time frame for a model is specified as:
- a Model Start Date, which can be either a year (implying 1 Jan), or a fully qualified date if you want to model in financial years, e.g., from 5 Apr 2001
- a Model Run Length, measured in years from the Model Start Date
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an Include Year Zero flag, which governs the inclusion of a base or reference period immediately preceding the Model Start Date.
Although the new model engine can process any arbitrary series of periods (measured in days), the current interface only provides for a number of initial Years in Quarters, or a number of initial Quarters in Months, commencing from the Model Start Date. Shorter time periods are not available for year zero because this is now designed and reserved as a base or reference period.
Typical model run period
The model engine calculates a full set of results for each specified period, working on the general principle that equipment is installed at the beginning of a period to match end-of-period demand. For a given period, demand assumptions are evaluated from parameterised input data according to the number of elapsed days to the end of the period, whilst tariff and cost trend data are evaluated at the beginning of the period.
Year zero
If you choose to include year zero in a model, it always appears as a single period with no sub-annual detail, because it is designed and reserved as a base or reference period which allows you to capture initial conditions. Results are still calculated for year zero so that initial equipment installation matches initial demand.
Pre-Run Data
A number of model inputs specify various pre-run data. Two of these – namely Resource Installation Profile and global Prior Annual Loss – are time-series inputs which allow you to fix particular values in time. The remaining pre-run data, such as Service Initial Creditors, Service Initial Debtors and global pre-run inputs for the balance sheet, are all scalars, i.e., single numbers with no explicit time qualification. These individual figures are always interpreted as initial conditions for the first period of the model run. So if you set the run-period input Include Year Zero = Yes, the figures are associated with the beginning of year zero; otherwise they relate to the end of year zero, i.e., the beginning of year one.
Note: If these data were always identified with the beginning of year zero, then they would be detached from a model starting in year one. Conversely, if they were always identified with the end of year zero, then they would conflict with the actual results calculated for year zero.