In dct:PeriodOfTime, add new properties:

• time:hasBeginning: Beginning of a period or interval
• time:hasEnd: End of a period or interval

The values of time:hasBeggining and time:hasEnd are time:Instant for whose position several options are available ( times that do not use the conventional Gregorian calendar, geological and archeological periods, and times given as numeric positions on a time-line.)