The dividing date is a new term introduced within NEC4 contracts. Whilst it is not capitalised and therefore not a "defined term" in clause 11, it is described quite well within clause 63.1 as to what it is.
The dividing date is the switch point between using actual Defined Cost and forecast Defined Cost of a compensation event. It states that if the CE has arisen from a Project Manager instruction, the date of that instruction is the "dividing date" (i.e. the CE will only ever be assessed as a forecast as the Contractor should not have done any work prior to the instruction). For any other compensation events not arising form a PM instruction, it states the dividing date is the date of the notification from the Contractor that it is a compensation event.
The dividing date is the switch point between using actual Defined Cost and forecast Defined Cost of a compensation event. It states that if the CE has arisen from a Project Manager instruction, the date of that instruction is the "dividing date" (i.e. the CE will only ever be assessed as a forecast as the Contractor should not have done any work prior to the instruction). For any other compensation events not arising form a PM instruction, it states the dividing date is the date of the notification from the Contractor that it is a compensation event.