  Fast timing capability in X-ray observation of astrophysical objects is one
of the key properties for the ASTRO-H (Hitomi) mission. Absolute timing
accuracies of 350 micro second or 35 micro second are required to achieve
nominal scientific goals or to study fast variabilities of specific sources.
The satellite carries a GPS receiver to obtain accurate time information, which
is distributed from the central onboard computer through the large and complex
SpaceWire network. The details on the time system on the hardware and software
design are described. In the distribution of the time information, the
propagation delays and jitters affect the timing accuracy. Six other items
identified within the timing system will also contribute to absolute time
error. These error items have been measured and checked on ground to ensure the
time error budgets meet the mission requirements. The overall timing
performance in combination with hardware performance, software algorithm, and
the orbital determination accuracies, etc, under nominal conditions satisfies
the mission requirements of 35 micro second. This work demonstrates key points
for space-use instruments in hardware and software designs and calibration
measurements for fine timing accuracy on the order of microseconds for
mid-sized satellites using the SpaceWire (IEEE1355) network.
