The MTBF numbers are generated by dividing the total uptime by the number of unscheduled interrupts. This is time that is scheduled for CCS staff and the vendor to use the machine. Users sometimes use the machine during this time, but the machine is subject to pre-emptive use by vendor and CCS staff.
Shutdown is a scheduled period of unavailability, generally for hardware upgrades.
The machines are not accessible to users at all during shutdowns. This time is not part of uptime. All numbers were obtained from reports generated by the machine vendors. The vendors have stated that the numbers in those reports may not be totally accurate.