Maybe we should implement fixed delay correctly and do run a one-time on-upgrade migration where we change all existing timer scripts to fixed rate, since that’s what they are in reality… 
edit: no, then you’d lose the safety guarantees of fixed delay.
Maybe Java should stop stealing your features after making you think you had them for years! 
Any reason you can’t terminate each execution of the script with a callback that schedules another one in x miliseconds?
(Note that I don’t actually need this feature so please do whatever you want with it)
Well we could, but there’s little reason to do that instead of using ScheduledExecutorService.
The big issue here is an unexpected behavior change on upgrade for all the existing timer scripts out there, not how to actually implement fixed delay.
Good luck! 
I think you should just fix it. Anyone who chose fixed delay operation was expecting variable start times. That they’ve been getting earlier starts than “proper” is simply a variety of “variable”.
I vote that Ignition fixes it as it is described in the HELP.
Basically it has been functioning like the HELP says, but there is LESS time between executions than there should be.
The fixed delay behavior will be fixed in 8.1.2.