What do you mean by IPL time limits? Do you mean power down time limits?
Try this link: http://publib.boulder.ibm.com/infocenter/iseries/v5r4/topic/experience/ipla3.pdf
I believe I found the answer….. Since I’m using BRMS to perform a full system save, I can go into the BRMS System Policy and change the IPL Controls to indicate earliest and latest allowable IPL time.