//base: Time-Related Functionality

This directory contains the portions of //base that deal with time-related concepts. Most critical are the classes in time.h.

• Time represents a specific wall-clock time. It is computed from the system clock, meaning successive requests for the current time might not always monotonically increase (e.g. across automatic or manual clock adjustments). Generally it is appropriate for anything human-visible, e.g. the last modified date/time of a file or a future time when Chrome will be automatically restarted, but users must safely handle negative durations and other effects of the non-monotonic clock.
• TimeTicks is computed from an incrementing counter. It thus increases monotonically, meaning it's usually appropriate for determining how much time elapses between two nearby events, e.g. for function timing for profiling, or to schedule a task "100 milliseconds from now", regardless of what the clock reads at that point. However, its behavior across power-saving mode changes is platform-dependent, meaning it may not increment during times when the system clock continues to run, and the precise conditions under which it does increment vary by platform. This usually makes it inappropriate for long durations, especially in cross-platform code; for example, a histogram that uses TimeTicks to count events in a thirty-day window will show very different results on a platform that pauses the counter during sleep compared to one where it continues to run. It is also non-sensical to try and convert a TimeTicks to a Time and then use that as a reference point for any other TimeTicks value, since even within the same process, both intervening sleeps and intervening clock adjustments may mean the values should have had different reference points.
• TimeDelta represents a duration between two Times or TimeTicks.

There are also various files dealing with clocks, which are primarily useful when tests need to modify how the program tracks the passage of time. See /base/test/task_environment.h's MOCK_TIME ability for testing components which post tasks.