This read-write DisplayString holds the date the UPS batteries were last replaced, in mm/dd/yyyy format, originally set at manufacture and meant to be manually reset after a real battery replacement. It is a maintenance-tracking field an admin must update themselves whenever the physical batteries are swapped, since the UPS can't detect that on its own. For example, an admin sets this to "08/21/2026" right after installing new battery packs so future age calculations start from the correct date.