Remaining Life of 'From Prorate Date Years'

Hi everyone

Do you know why the remaining life of the 'From Prorate Date Years' is added up one period?

I have prorate dates set up on the next month as below.

From          To                    Prorate Date

1-JAN-17      31-JAN-17     1-FEB-17

1-FEB-17     28-FEB-17     1-MAR-17

1-MAR-17     31-MAR-17     1-APR-17

1-APR-17     30-APR-17     1-May-17

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I know the prorate date is in the next month. But I just need the definition or the clarification of this.

If you also could provide any document id, it would be very helpful.

Thank you