SystemcurrentTimeMillis

System.currentTimeMillis is a public static method in the Java standard library (java.lang.System) that returns the current time in milliseconds. The value represents the number of milliseconds since the epoch, defined as 1970-01-01T00:00:00Z (UTC). It is commonly used to obtain timestamps for logging, metrics, or converting times for storage or display.

The returned value reflects the host system clock, i.e., wall-clock time. Consequently, it can be affected by manual clock changes or time synchronization services (such as NTP). If the system clock is adjusted forward or backward, successive calls may not be strictly increasing, and the value can jump.

Typical uses include generating timestamps for logs, calculating elapsed time by taking differences between two readings, or interfacing with APIs and data formats that require epoch milliseconds. The method returns a long, making it suitable for representing a wide range of dates and times.

Implementation notes emphasize that System.currentTimeMillis is a native method whose exact behavior depends on the platform and JVM. It is not designed for precise elapsed-time measurements due to potential clock adjustments and synchronization.

Alternatives and related APIs: For monotonic elapsed time, System.nanoTime provides a high-resolution timer that is not related to wall-clock time. For modern date-time handling, the java.time API offers Instant.now() and related methods, with conversions such as Instant.now().toEpochMilli() or Clock.systemUTC() for controlled time sources.