Suppose being charged by time used to run your tasks. Then your task takes -1 second, you will be charged zero, 1 second, or 18 quintillon seconds?
When a negative leap second is applied it just means that 23:59:59 is skipped. The seconds go 56 57 58 0 1 2 3.
vs a positive leap second where time goes 56 57 58 59 60 0 1 2 3.
So if you are billed by time and it's tracked via a timer then your time is still accurate but if it's tracked by reading your start and end time then your billing will just read an extra second.
Positive leap seconds are harder as they require you to reason about a clock that includes a 60th second but systems view negative leap seconds as if nothing happened for an entire second.
Now there are some cases where this runs into issues. If you have jobs that fire off based on the time then you can get jobs running a second early. Normally this shouldn't be a problem but in some cases this could result in contention of resources.
You could also accidentally trigger watchdogs but any watchdog worth its salt will use a real timer not wall time.
But overall negative leap seconds are way easier as they are still monotonically increasing and don't require downstream systems even understanding the concept of a leap second in the first place.
Well, this seems to touch heavily on the concern: if your job normally takes 0.1s and costs $100, it could accidentally be considered to have taken 1.1s and therefore cost $1100. This is quite contrived, but if you consider that some people put very infrequent cron jobs at the top of the hour (which isn't typically the best idea, but I digress) and it might start just ahead of what the billing system considers to be the top of the hour, theoretically there's something to this concern.
And tbh if you are performing ops at a scale where that level of expense is present it's nobody's fault but your own if your orchestration is cronjobs on the 58th second of the 59th minute of the last hour of the year.
And at any time scale past the order of seconds, this is a small enough time jump that it's a rounding error in ops expenses.
A leap second is added. The system goes "oh shit, I'm a second ahead", and subtracts a second. And that's how you get a negative duration and exactly this problem.
Whereas if a leap second is subtracted, you get charged for an extra second.