# Time calculations
An observation from an indicator species has a base duration of 60 days. These days are defined as 30 days prior, and 30 days post the documented observation date-time stamp. The unit by contrast has a base duration of one month, or 30 days. So one observation issues at most *two* credits, one before and one afterward.
This time period was chosen carefully based on the periodicity of subsistence lifestyles for IP and LC experts, the cost of monitoring devices for IP and LC, the incompletely characterized effects of electromagnetic fields from tracking devices on the full gamut of species within protected zones, and the potential for hunting or poaching to reduce animal populations during the monitoring period ([Appendix H](/appendices/appendix-h-indigenous-authors.md)).
The system does not allow double-crediting, for observations overlapping in time. Observations occur fluidly throughout the monitoring period. Monitoring groups exhibit hunter-gatherer periodicity in work activities. Furthermore, animals have diurnal and seasonal variation in observed behaviors. The methodology accounts for this, summing observations to standardize crediting in both space ([Figure 9](https://isbm.savimbo.com/calculation/pages/0kitnsjozpAvA46xKYmB#figure-9.-union-of-observations-which-overlap-in-time-and-space-to-calculate-crediting)), and time ([Figure 10](#figure-10.-biodiversity-credits-appearing-and-disappearing-over-time-with-different-species-observat)).
1. The circle(s) representing the credited hectares for the observation points are first assigned a date-range, which may overlap in area with another observation point during the same time period,
2. If observations overlap in time, the area of the observations are unioned for each date,
3. The resulting map is then clipped by the project boundaries to calculate the area to be credited,
4. The time period and map are then summed to present the total number of hectares that are available for crediting within that year.
#### Figure 9. Union of observations which overlap in time and space to calculate crediting
Just as we credit only once for one hectare, even if more than one species is sighted, we credit only once per day for the hectare, even if there were multiple sightings that occurred with overlapping time periods. For example, if a sighting happened one day after a previous sighting in the same area, only one more day would be accounted for, not 30 days prior or after, because the delta is only that one extra day. This function in mathematics is known as the union of overlapping sets.
#### Figure 10. Biodiversity credits appearing and disappearing over time with different species' observations
Figure 10. Biodiversity credits appear and disappear over time with observations of different species.
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