The time span field is based on the As of Date.  This can be tested by changing the time span to 7 days.  You will notice that only KPIs with an as of date in the past 7 days will appear.
When you create a KPI you generate values for the KPI instance you generate the As of Date.  You have the option to view data based on the current date or another date.
If you select Process once using current date, the program creates one history record relative to the current date.
Note: We recommend you back date values for new KPI instances, when trend information does not exist. After you create a KPI instance, you can schedule a business process to update its values. Each time the process runs, the program creates a new KPI instance history record for the values of the date the process runs.   
If you select Back-date, the program creates several history records for the time period determined by the selected interval such as “every 3 months” or “every 7 days”. For example, if you back date a KPI for a year with an interval of every three months, the program create four history records, one of each quarter.
If you select Back-date, enter the start and end dates for which to view data. In the Calculate value every field, select the interval by which to generate data.
You can update the As of Date by updating the KPI values as follows:
  1. From Analysis, click KPIs. The KPI Instances page appears.           
  2. In the grid, select the instance for which to generate values.      
  3. On the action bar, click Update KPI value. The Update KPI screen appears.
  4. Select whether to view data based on the current date or another date.