Mt 16:21 From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
Now, what was happening around Matthew 16:21? It is just after Simon Peter had declared 'Thou art the Christ, the Son of the living God' and, in the next chapter, the transfiguration. Jesus is still in Gallilee, then he goes to Capernaum, then he makes his way to Judaea for the last time, for in chapter 21, the triumphal entry into Jerusalem is described.
This declaration of Peter seems to be mirrored in John 6:68 - just after Jesus walks on the sea, as it is in Matthew. So, if you look in John, what comes next is a Jews' feast - of Tabernacles - in ch.7:2 - and shortly after that comes the time leading up to his death, at Passover. Therefore, we can put his predictions of his imminent demise at around six months before his passion.
thats because the birth, death & resurrection of Christ was long ago foretold by prophets in Old Testament scripture... <Isaiah for one> but since Christ only lived 31 years and only the last 3 did He preach & minister to His disciples and believers - THAT was a very short time.... how close an estimation were you hoping for???