Luke never said "29 AD." You inserted it.

Luke simply said the "15th year of Tiberius." Now, listen up:
Tiberius became
sole emperor on the death of his adoptive father, Augustus, on 19th August AD 14. However, it is a well-known fact that he had become co-regent with his ailing father two years earlier in
AD 12.
In that year, he was made supreme military commander over Caesar's armies and provinces.
Ancient coins from Antioch dated AD 12 display the head of Tiberius and documents attest to his reign being fully in force from that time. Thus, his inauguration in AD 14 as emperor was only a formalisation of a reign that had begun two years earlier.
Therefore, the fifteenth year of the reign of Tiberius would make it late
AD 26 correlating with the baptism of Jesus and supporting the
AD 30 date as the most likely date of his crucifixion.