Why invalid cron fails silently
Crontab does not compile your line. A 99 in the minute field never matches, so the job never runs and nothing logs a syntax error.
The five fields
Minute 0–59, hour 0–23, day of month 1–31, month 1–12 or JAN–DEC, day of week 0–7 or SUN–SAT. Steps, lists, and ranges are allowed. `*/0` is not.
Unix vs Quartz vs AWS
Six fields or a `?` is a different dialect, not a broken minute. The converter maps those strings to Unix when the mapping is honest.
L, lists, and steps
This validator accepts the CronFox capability matrix, including Sunday 0/7 and fixture-gated day-of-month L/W. A library accepting a token is not enough to mark it deployable.
After it is valid
Preview upcoming times in the calculator, or schedule the HTTP job on CronFox with retries and execution history.