Quartz Scheduler
An open-source job-scheduling library for the JVM whose extended cron grammar with seconds, year, and AND-semantics for day fields is widely copied by other schedulers.
Quartz Scheduler is a Java job-scheduling library originally developed by OpenSymphony and later maintained under Terracotta and the Quartz Enterprise Job Scheduler project. It is embedded in countless Java backends, Spring applications, and enterprise schedulers, and its cron grammar has been ported as Quartz.NET for the .NET runtime. Quartz extends the classic five-field Unix cron syntax in several ways. Expressions have six required fields — seconds, minutes, hours, day-of-month, month, day-of-week — plus an optional seventh year field (1970-2099, depending on version). Day-of-week is numbered 1-7 with Sunday=1 (or SUN-SAT), unlike Vixie Cron where Sunday is 0. Quartz also adds the special characters `L` for the last day of the month or week, `W` for the weekday nearest a given day-of-month, and `#` for the nth weekday of the month (`6#3` is the third Friday). Critically, Quartz uses AND-semantics between day-of-month and day-of-week, the opposite of Vixie cron's OR-quirk. To force an unambiguous expression, Quartz requires exactly one of the two day fields to be the `?` placeholder, meaning "no specific value". This eliminates the ambiguity that has bitten generations of Unix sysadmins. Quartz expressions are evaluated in the trigger's configured timezone rather than always UTC, which is one reason many Java applications prefer it over operating-system cron.