nettools does not expose the original lease lifetime. It's a missing
API. Instead, it only exposes the timestamp when the lease will expire.

As a workaround, we calulate the timestamp by subtracting the current
timestamp from the expiration timestamp, assuming that the lease was
received just now. However, it was not received *exactly* now, but a
few milliseconds before. Hence, the calculated timestamp is not exact
here and likely a few milliseconds less then the actual (full integer)
value.

Account for that by rounding the value to the second.