launchd StartInterval

Good day! I am stumped. I have a launchd plist that has a StartInterval set for 86400 (once per day). I do not want to convert to a StartCalendarInterval but I would like to shift the execution time by approximately twelve hours. I cannot figure out where launchd stores the time for the next execution of a particular StartInterval plist or all jobs. Some posts refer to a "diary," but I have found no such thing. I have tried launchctl stop followed by launchctl start but there is no change to the time it runs each day. Any ideas? Thanks, Charlie