This directory contains the portions of //base that deal with time-related concepts. Most critical are the classes in time.h. Time and TimeTicks both encode absolute times, but TimeTicks is monotonic and should be used for most internal purposes, while Time can move backwards and is primarily for human-readable times. TimeDelta is a duration computed from either of the above concepts.
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.