If we have a khums date fixe can we change it to a later date in the same year?

Or is it possible that we take out the khums on the money that we have and take out the khums of the possessions on a later date in the same year because we can't determine the khums amount of the unused possessions and the time has come?

Yes, you can change the date by paying the khoms of the amount in hand on due date, and then pay the khoms of the gross amount on the new date.