Userspace exposed time units
zx_time_t is in nanoseconds.
Use zx_clock_get_monotonic() to get the current time as the number of nanoseconds since boot.
Kernel-internal time units
lk_time_t is in nanoseconds.
When writing code that will run in the kernel, to read the current monotonic clock value, use:
#include <platform.h>
lk_time_t current_time(void);