Several active tasks use a special kind of timestamp called a "device timestamp" to track events within the task. Device timestamps indicate elapsed seconds since the device was powered on.
For example, the Stroop Active Task uses device timestamps for the StartTime and EndTime fields, to mark the time between the prompt and the response.
When using device timestamps, the absolute time is not relevant, only the relative difference between timestamps. In the example above, there was a reaction time of about 1.2 seconds.