Do not assume that the kernel boot time is invariant. It is not. FreeBSD
uses the formula:
wall_time = boot_time + uptime
where uptime is monotinically increasing and boot_time is adjusted to
get the proper time of day. FreeBSD offers a way to retrieve the uptime
directly, so use that instead of trying to compute it by subtracting
boot_time from wall_time.

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To test, one can step the time after starting bsnmpd. Since boot time changes but the new value isn't re-fetched, the uptime will step when it shouldn't.
With the fixed code, this doesn't happen.