[SalesForce] What happens to scheduled jobs set to half past midnight, when the clocks go forward or back

Rather than scheduling a one-time job, schedule a recurring job.

Schedule the job to run on an hourly interval (every hour). As part of the finishing phase of your job, cancel this hourly schedule and replace it with another similar hourly schedule where the first execution is set to be a short period (let's say 5 minutes) from the finish of the job.

This works in a very similar way to using a "one off" schedule (as per your existing implementation) - in both of these implementations the job is rescheduled in the finish phase, but by using a recurring schedule you have the added benefit that if for any reason the job does not execute, the platform will attempt to run it again an hour later, and every hour until it succeeds.

Note that we don't know why the job may fail to execute - but we're assuming that it relates to platform maintenance. Chaining one-off scheduled jobs together relies on the successful start and completion of each job for the integrity of the chain, whereas using a recurring scheduled job provides "auto-resume" behaviour regardless of the successful start / completion of an individual job.

Example process flow:

(1) at 12:00 we schedule a job to run every every hour, at 5 minutes past the hour: 12:05,13:05,14:05...etc...

(2) at 12:05 the batch manager job is started according to the hourly schedule, and this checks your custom batch job object records to see if there is any work currently running or waiting.

It finds that there are no jobs running but there is a job waiting: "Foo". The batch manager therefore starts the batch process for Foo.

(3) at 13:05 the batch manager job is started according to the hourly schedule.

On this occasion it finds that job Foo is in progress and so quits taking no action.

(4) at 13:35 job Foo finishes.

In the finish phase, the existing hourly scheduled job is cancelled, and another new hourly job is scheduled, this time to run at 40 minutes past the hour: 13:40, 14:40, 15:40...etc…

(5) at 13:40 the batch manager job is due to start according to the hourly schedule, but this fails (we assume because of platform maintenance)

(6) at 14:40 the batch manager job is started according to the hourly schedule.

It finds that there are no jobs running but there is a job waiting: "Bar". The batch manager therefore starts the batch process for Bar.

etc.

I haven't seen that but when I am debugging scheduled batches, I typically schedule it via the dev console to run in 2 minutes time, and then watch the logs - even if if something isn't written to the jobs queue, you should see it in the logs for the running user (which would be you if created via Dev Console)