PRE_TBTT_USEC
if (tbtt <= PRE_TBTT_USEC) {
time = (tbtt - PRE_TBTT_USEC) * 1000ull;
if (tbtt < PRE_TBTT_USEC)
time -= (PRE_TBTT_USEC - tbtt) * 1000ull;
time += (tbtt - PRE_TBTT_USEC) * 1000ull;