X_CRON_NOT_DESCRIBABLE
Means
a valid cron expression describeCron has no vocabulary for
Typical cause
a 6-field expression whose seconds field says something a 5-field one cannot (*/10 * * * * *, 30 0 3 * * *) — CronPhrases has no seconds phrase, so the summary would be wrong
Fix
render the real runs with nextCronOccurrences(expr, zone, from, 3), or describe the 5-field schedule the fix: line prints — never a summary that drops the seconds