Welcome to the E-Prime timing information area.
E-Prime reports timing accuracy to the millisecond precision level. Although E-Prime coordinates with the operating system and external devices to continually have control of the computer's execution, the operating system or other devices can take control at any time, which will cause an experiment to be suspended while the operating system task is completed. When these instances occur, E-Prime continues to accurately timestamp so that post-analysis can filter the trials correctly.
As stated above, E-Prime reports timing accuracy to the millisecond precision level. PST has conducted formal timing tests on Windows XP, 7, 8, and 8.1 (KB 5628 - E-Prime 2.0 Timing Data). For more information, please consult the Critical Timing Chapter in the E-Prime documentation.