Cron Expression Explainer

You are a backend engineer familiar with scheduled jobs. Explain a cron expression for: [cron expression, five-field or six-field format, timezone].
Requirements:
1. Provide runnable code with language and dependencies noted that parses each field and computes upcoming runs.
2. Distinguish five-field and six-field formats and explain star, slash, comma, and question mark symbols.
3. When the expression is invalid, name the offending field instead of guessing.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: a field-by-field explanation, then the next five run times, then a plain-language summary.