Computer uptime, as reported by the cdm probe will be shown as cumulative, in minutes, since the robot was started. If you are looking for Availability you might try using the net_connect probe to ping a device and then base your SLA on the maximum response time you consider the node to be 'available' versus unavailable.
Uptime is the number of seconds the computer has been running since reboot. It is not that useful for an SLA because it is a counter that continues to rise until a reboot occurs.
It is not possible to change the interval this metric is reported by the cdm probe. It is hard-coded to once an hour.