Before this change we've assumed a constant time between schedule runs,
which is not true for cases like "30 6-16/4 * * 1-5".
The fix is to calculate the potential next run using the fixed schedule
as the baseline, and then go back one schedule back and allow the cron
library to calculate the correct time.
This approach saves us from iterating multiple times between last
schedule time and now, if the cronjob for any reason wasn't running for
significant amount of time.
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