Epoch

Enhanced Definition

An epoch, in computing, refers to a fixed point in time from which system time is measured. While z/OS has its own native timekeeping mechanisms, the term "epoch" is most commonly encountered on the mainframe when interoperating with distributed systems (like Unix, Linux, or Windows) that typically use the **Unix Epoch** (January 1, 1970, 00:00:00 Coordinated Universal Time - UTC) as their time origin. In the context of z/OS, particularly within **z/OS UNIX System Services (USS)**, the epoch refers to a fixed point in time from which system time is measured. It is defined as **January 1, 1970, 00:00:00 Coordinated Universal Time (UTC)**, and time is typically represented as the number of seconds that have elapsed since this epoch. This standard aligns z/OS UNIX with the **POSIX** and **UNIX** timekeeping conventions.

Key Characteristics

- Reference Point: It serves as a universal starting point for measuring time, allowing for consistent timestamping across different systems and locales.
- Unix Epoch: The most prevalent epoch is January 1, 1970, 00:00:00 UTC, with time typically measured in seconds or milliseconds since this point.
- z/OS Native Time: z/OS's hardware clock, the Store Clock (STCK), has its own epoch of January 1, 1900, 00:00:00 UTC, measuring time in microseconds. Conversions are necessary when integrating with Unix epoch systems.
- Integer Representation: Epoch values are typically stored as signed or unsigned integers, counting the number of time units (e.g., seconds) that have elapsed since the epoch.
- Year 2038 Problem: For 32-bit signed integer representations of the Unix epoch, the maximum value will be reached in 2038, potentially causing issues similar to the Y2K problem.

Use Cases

- Interoperability: Exchanging timestamped data between z/OS applications (e.g., COBOL, CICS, DB2) and distributed systems that rely on the Unix epoch for their timestamps.
- Data Archiving & Logging: Storing timestamps in a standardized, platform-independent format within logs, audit trails, and historical data to ensure consistency across heterogeneous environments.
- API Integration: When z/OS applications consume or provide web services or APIs that specify timestamps using epoch values, requiring conversion routines.
- Time Calculations: Performing calculations like duration, age, or scheduling based on a common, unambiguous reference point, especially when integrating with non-mainframe systems.

Related Concepts

STCK (Store Clock) / TOD (Time-of-Day) Clock: z/OS's fundamental hardware clock, which counts microseconds from January 1, 1900. Applications often need to convert between STCK values and Unix epoch timestamps for cross-platform compatibility.
GMT/UTC: Epochs are almost universally defined relative to Coordinated Universal Time (UTC) to eliminate ambiguities caused by local time zones and Daylight Saving Time.
COBOL DATE and TIME Intrinsic Functions: While COBOL provides intrinsic functions for date and time, converting to/from epoch timestamps often requires custom routines or Language Environment (LE) services.
**C Language `time_t