It's simply a given date. To absolutely date an event, physico-chemical
methods are performed in the laboratory on samples taken from the field.
Radiocarbon dating (Carbon-14) is the best known method. It was used at Ishango,
aswell as the racemisation of amino acids.
Events are placed in relation to each other without giving a date.
In archaeology, palaeontology and geology, stratigraphic observations are
used to locate events.
The basement consists of layers made by water, wind, man, volcanic eruption etc.
The layer on which we walk is the last to have formed, while the deeper layers
are older, just like the artefacts they contain. In Ishango, both stratigraphy
and laboratory methods were used for dating.