In general semantics, the dates are a kind of index. Their interest and use are similar to that of indexes. This tool enables us to remember that things, people, theories, etc., change over 'time'.

Examples:

We can say for example: "Scientists₁₈₉₀ believed in the existence of ether", "Smith₁₉₇₀ was communist", "Capitalism₁₉₀₀ is not capitalism₁₉₉₇", etc.