I find "absolute + Japanese" to be confusing to the point of contradictory.

It could be so easy:

• absolute means: one period within which you have to play all your moves
• overtime means: a subsequent period within which you need to play a certain number of moves; there are basically two kinds: a multiple, shorter, overtime for a single move (Japanese overtime) and a single, longer, overtime for multiple moves (Canadian overtime).

Most overtime systems come after a first period which we could call "main time" but certainly not "absolute".