解析Cron表达式，计算下次执行时间并转换为易读格式。
分钟 小时 日（月） 月 日（周）
A cron expression is a string consisting of five or six fields that define a schedule for running automated tasks. Originally from Unix cron daemon, this syntax is now used in many scheduling systems. Each field represents a time unit: minute, hour, day of month, month, day of week, and optionally year.
Standard cron has 5 fields: minute (0-59), hour (0-23), day of month (1-31), month (1-12), day of week (0-6, Sunday=0). Special characters include: * (any), , (list), - (range), / (step). Extended formats may include seconds and year fields.
0 0 * * *
每 5 分钟
*/15 * * * *
每分钟
0 9 * * 1-5
每天
0 0 1 * *
First day of every month at midnight
Traditional cron uses the system timezone. Modern schedulers like Kubernetes CronJobs let you specify the timezone. Always verify which timezone your cron system uses.
Standard 5-field cron doesn't support seconds (minimum is every minute). Some systems like Quartz support 6-field cron with seconds. Alternatives include using sleep loops or task schedulers designed for sub-minute intervals.