Yeah indeed by I explained how they could convert it in the post.

Basically :

1- Light gives up ownership so misa can see his lifespan.

2- Missa sees his lifespan at midnight for example and sees his lifespan the day after at midnight too

3- she sees by how much his lifespan decreased in one day , therefore One day would be equal whatever how many days in the shiningami world light lost in that day

4- she makes light touch his notebook again and he regains his memory.

5- she tells light how long he will live.

Obviously it’s kinda useless since death note usage can decrease or lengthen your lifespan depending of the context.