GMT - Greenwich Mean Time

Enhanced Definition

Greenwich Mean Time (GMT) is a historical time standard based on the mean solar time at the Prime Meridian (0° longitude) in Greenwich, London. In the mainframe context, it served as a fundamental reference point for system clocks, timestamps, and time zone calculations, particularly before the widespread adoption of Coordinated Universal Time (UTC).

Key Characteristics

- Historical Reference: Historically used as the primary time standard for computing systems, including IBM mainframes, before UTC became the preferred standard.
- Zero Meridian Basis: Defined by the mean solar time at the Greenwich meridian, serving as the basis for calculating all other time zones globally.
- System Timestamps: Many older z/OS system components, logs (SMF records, SYSLOG), and application timestamps were historically recorded or referenced in GMT.
- Time Zone Foundation: Provides the foundation for deriving local time zones by adding or subtracting offsets (e.g., GMT+1, GMT-5).
- No Leap Seconds: Unlike UTC, GMT does not explicitly account for leap seconds, which can lead to minor discrepancies over long periods, though mainframe systems typically align with external time sources that manage this.

Use Cases

- System Logging and Auditing: Timestamping events in SMF records, SYSLOG, and other system activity logs to provide a consistent, global reference for auditing, problem determination, and security analysis.
- Job Scheduling: Defining precise start and end times for batch jobs and automated tasks, especially in environments with distributed systems or across different geographical regions.
- Data Synchronization: Ensuring consistent time across multiple LPARs or sysplex members, or between mainframe and distributed platforms for data replication, backup, and recovery operations.
- Application Development: COBOL or PL/I programs might use system calls to retrieve the current time, which historically could be GMT or local time, requiring developers to understand the context.
- Global Operations: Facilitating coordination of operations, data transfers, and system maintenance windows across geographically dispersed data centers.

Related Concepts

GMT is closely related to UTC (Coordinated Universal Time), which is the modern, more precise international standard for timekeeping that z/OS systems primarily use today. While often used interchangeably, UTC accounts for leap seconds, making it more accurate. GMT serves as the base for calculating local time zones and is fundamental to STP (Server Time Protocol), which synchronizes time across z/Systems within a sysplex to ensure all LPARs maintain a consistent UTC time.

Best Practices:

Prefer UTC for New Development: For all new z/OS applications and system configurations, always use UTC as the preferred time standard due to its precision and international acceptance.
Understand Time Zone Conversions: Be acutely aware of the differences between GMT/UTC and local time, especially when developing user-facing applications or interacting with external systems.
Leverage STP: Ensure Server Time Protocol (STP) is correctly configured and monitored within your sysplex to maintain highly accurate and synchronized UTC time across all z/OS instances.
Consistent Time Sources: Configure z/OS to synchronize with reliable external time sources (e.g., NTP servers) to prevent time drift and ensure accuracy.
Document Time Standards: Clearly document whether GMT, UTC, or local time is used for timestamps in logs, databases, and application data to avoid confusion during analysis or debugging.