Interpreting Decision Time (DT) in temporal databases
I don't know if there is necessarily a decision period, but there might be a decision date, separate from the valid period start date and the transaction date, where it makes sense to indicate when exactly a decision was made to make a new entry valid.

In your example the decision to hire Okihara probably had been made at some time prior to '2014-01-01', in which case it might have put certain obligations on the employer even before Okihara's first day on the job, so it made sense to record it separately. 

One might presume that the decision date will never be after the transaction date, while the valid date might still be arbitrary.