# Help:Property constraints portal/Contemporary

Two entities linked through a property with contemporary constraint (Q25796498) must be contemporary, that is, must coexist at some point in history. Properties with this constraint are called contemporary properties.

There are two main categories of contemporary properties:

1. Those requiring use, synchronous communication or direct physical interaction. For instance, uses (P2283), doctoral advisor (P184), conflict (P607), spouse (P26), etc.
2. Those describing the relative location. For instance, located in the administrative territorial entity (P131), country (P17), located in or next to body of water (P206), location (P276), etc.

If, for practical reasons, this constraint is used in other properties, then it should never be defined as mandatory and a list of exceptions should be managed.

## When are two entities contemporary?

Formally, two entities ${\displaystyle e_{1}}$ and ${\displaystyle e_{2}}$ are contemporary if, and only if, their life intervals intersect, ${\displaystyle {\mathcal {I}}e_{1}\cap {\mathcal {I}}e_{2}\neq \emptyset }$.

The life interval ${\displaystyle {\mathcal {I}}e}$ of an entity ${\displaystyle e}$ is the closed interval bounded by its start time and its end time, ${\displaystyle {\mathcal {I}}e=[e_{min},e_{max}]}$.

The start time ${\displaystyle e_{min}}$ is the minimum acceptable value of all statements on ${\displaystyle e}$ using start properties date of birth (P569), inception (P571), start time (P580) or point in time (P585). Values with deprecated rank are ignored. If there are no valid statements using these properties, then ${\displaystyle e_{min}=-\infty }$.

The end time ${\displaystyle e_{max}}$ is the maximum acceptable value of all statements on ${\displaystyle e}$ using end properties date of death (P570), dissolved, abolished or demolished (P576), end time (P582) or point in time (P585). Values with deprecated rank are ignored. If there are no valid statements using these properties, then ${\displaystyle e_{max}=+\infty }$.

In boolean algebra (Q173183) and most programming languages, two entities ${\displaystyle e_{1}}$ and ${\displaystyle e_{2}}$ are contemporary if, and only if, ${\displaystyle e_{1min}\leq e_{2max}\land e_{2min}\leq e_{1max}}$.

### Violation

When two entities ${\displaystyle e_{1}}$ and ${\displaystyle e_{2}}$ are not contemporary, ${\displaystyle {\mathcal {I}}e_{1}\cap {\mathcal {I}}e_{2}=\emptyset }$, but are linked through a contemporary property, then there is a violation of the contemporary constraint (Q25796498).

## Possible actions

There are several possible ways to address a violation of this constraint:

• Most likely, change or remove the statement that links ${\displaystyle e_{1}}$ and ${\displaystyle e_{2}}$ through the contemporary property.
• Fix the time value ${\displaystyle min\{e_{1max},e_{2max}\}}$. This value should be later.
• Fix the time value ${\displaystyle max\{e_{1min},e_{2min}\}}$. This value should be earlier.

## More

Original work about the contemporary constraint (in Spanish)

The contemporary constraint was created, analyzed and implemented as an academic project by David Abián titled Contemporary constraint: definition and implementation in a knowledge base (doi:10.13140/RG.2.2.31671.57763), which includes deeper explanations about its nature and some mathematical properties that allow to measure the quality and the temporal falsifiability of a data source.

The full text is available in Spanish on Wikimedia Commons and an scholarly article that summarizes this text will be published in English in 2019 on the occasion of The 34th ACM/SIGAPP Symposium on Applied Computing (SAC '19).