So, the other day I wanted a couple unique things to happen at a specific time, on a specific day. It was a unique situation, so I used the certain Time (with optional date) and entered the full time/date, including the year (didn't check annually) .. thought things were OK, but the rule ended up not doing anything .. investigating what happened I discovered that when the date includes the year, it is failing to schedule itself and records the following error into the log:
errorjava.lang.RuntimeException: CronExpression '0 55 15 07 12 2022' is invalid. on line 7664 (method appButtonHandler)
workaround for now appears to be to always schedule things at least annually, of course that means if I ever forget to pop back in to remove a just one time event, I could get surprised the next year..