What did Zechariah prophesy when St. John the Baptist was born?

In Luke we read that filled with the Holy Spirit, praising God (Lk. 1:68-75) he prophesied of his son (Lk. 1:76-70),

And you, child, will be called the prophet of the Most High;
for you will go before the Lord to prepare his ways,
to give knowledge of salvation to his people
in the forgiveness of their sins,
through the tender mercy of our God,
when the day shall dawn upon us from on high
to give light to those who sit in darkness and in the shadow of death,
to guide our feet into the way of peace.

How was John the Baptist conceived?

He was conceived in the normal way. It was miraculous because Elizabeth was past her child-bearing years, and she had been unable to conceive even in her younger years.

When was John the Baptist born?

St. John the Baptist was born about six months before Our Lord. Scholars disagree as to the exact year in which Jesus was born, but it was probably between 6 and 4 B.C., though possibly as late as 1 B.C. Errors in reconciling the calendars and events of the ancient world, when dating from Christ’s birth was adopted in 525 A.D., accounts for Christ being born B.C., rather than 1 A.D.