Rusage is a subset of a Unix-style resource usage counter for the current
process and its children. The counters are always 0 on platforms where the
system call is not available (i.e., systems where getrusage() doesn't
exist).
Get returns the counters for the current process and its children,
subtracting any values in the passed in "since" value, or an error.
The Elapsed field will always be set to zero.
Subtract subtracts the items in delta from r, and returns the difference.
The Date field is zeroed for easier comparison with the zero value for the
Rusage type.