There should be no problem with your returning in September 2015. By law, six month or less is a casual visit outside the U.S. with no impact. Absence for more than a year will automatically result in loss of U.S. permanent resident status. However, you are a U.S. permanent resident. If you are not in the U.S. for more than ½ the year, it appears that your permanent residence is elsewhere. We know of persons denied entrance and another who has lost permanent resident status for long periods away from the U.S. separated by short periods of stay in the U.S.. We have never seen such enforcement with the absences about which you write. You should be all rights. However, If you may be outside the U.S. for more than six month, and definitely is there is any possibility that you will be out for more than one year, simply obtain permission to return to your residence in the U.S.. File USCIS Form I-131.