In Slope, time is processed in monthly increments and always starts with Time Index = 0 at the projection start date (as specified in the Projection). The time index increases by 1 for each month. Negative time index values represent months prior to the projection start date.

This means that for all model points that are in force at the start of the projection, the issue date or start date of those model points will have time index values less than or equal to 0 (before the projection start). Any new business issued during the projection or assets bought by dynamic investment will have issue dates or start dates greater than 1.

### Time System Variables

There are several system variables available in the Formula Builder related to time.

*Time Index* – Returns the time index value for the current variable calculation context. This value is not necessarily equal to the current projection period time index that is being processed (see more below).

*Current Date* – Returns the calendar month/year corresponding to the Time Index.

*Projection Time Index* – Returns the current projection period time index being processed by the outermost calculation engine loop.

### Example

The below example shows how the various time indexes relate to each other. Assumed with have 2 variables, Var1 and Var2 with the following formulas.

Var1:

Var2:

At time 0, the calculation engine will calculate the value of Var1. The Time Index and Projection Time Index are both 0 at this point.

Var1 is equal to Var2 at time T+2, so the calculation engine will now calculate Var2 and time T+2.

Var 2 is equal to the Time Index plus the Projection Time Index. The Time Index is now 2 since we are calculating the value at T+2. The Projection Time Index is still 0 though (the original time index from the starting point, not the time index of the current variable reference). This gives us a result of 2.

At the next calculation time (Time 1), the engine will again calculate Var1 which is equal Var2 at time T+2 (Time 3). So the calculation engine will calculate Var2 at Time 3. Now the Time Index is 3 and the Projection Time Index is 1, giving a result of 4.