I've been looking for some NTP server responses and I have always seen that T3=T2. Is this always true. Its a bore to do 32 bit calcs and the fractional part in a small micro, so if I can simplify the formula it would be great current Time = T4 +1/2 ( T2-T1 + T3-T4 ) = T2 + 1/2(T4 - T1 )