# Create a Time Span Field

A time span field is a measure that calculates the time difference between two date/time fields (or a date/time field and a specified date).

To create a time span field:

- Select
**Cross > Define Field**. The**Define Fields**window displays. - Click
**Time Span**. The**Time Span Field**window displays.

Enter the following details:

Field Description **Field Name**Enter a name for the field. **Number of**Select whether to measure the time difference in **days**,**months**or**years**.**Between**Select the start and end points of the time span.

- Only date/time type fields will be available for selection.
- For the end point, instead of selecting a field you can optionally specify a fixed date.

Click

**OK**.

### Note About Time Calculations

By default, SuperSTAR uses an estimated method for calculating the difference between dates.

If you request that the difference between two dates is specified in years then the value is calculated by dividing by 365.25 (i.e. the effect of leap years is averaged out). This can result in inaccuracies if you calculate a range UDF on top of a time span UDF for dates that are on exact year boundaries. In this case the computed value will be slightly greater, or slightly less than the integer multiple of years depending on how many leap days fall into the range.

For example, if you calculate the days between 1/1/2001 and 1/1/2002 then SuperCROSS returns 365 / 365.25 = 0.9993, whereas a user (and the range UDF) would expect the result to be exactly 1 year.

As an alternative, SuperCROSS can use a different time calculation: discrete time calculation. When this mode is active, SuperCROSS computes, in addition to the total number of days, the integer value of the number of whole years, whole months and remaining days. Instead of averaging from the total number of days, it uses the following calculations:

```
datespan in years = nYears + (nMonths / 12) + (nDays / 365.25)
datespan in months = nYears*12 + nMonths + (nDays / (365.25/12))
```

In the example above, this calculation results in the expected whole number result for a full year.

To enable the calculation you need to create the following environment variable and set it to true:

`STR_DS_DISCRETE_TIME_SPAN=true`