When did Tisha B'av start. From the gemara & meforshim in taanis 12a it seems that it started before churban bayis rishon ....
Hello there and thanks for your inquiry.
The truth is that I'm not sure on what your question is based since you didn't specify which section of the Gemara you are referring to. I assume that you are referring to the fact that there was Tisha B'Av while the family of San'a ben Binyamin was bringing wood to the Beis ha'Mikdash.
First let me point out that this is clearly refering to sometime after the Second Beis ha'Mikdash was built, not the first. The Yerushalmi in Shekalim (Fourth Perek) quotes two opinions whether or not the celebration of Korban Etzim is relevant after the Beis ha'Mikdash was destroyed. This Beraisa is brought as proof to support the Amora who maintains that it does continue even after the Churban. If everyone else was fasting it must have been after the Churban and nonetheless this family continued to celebrate. (This is the way the Vilna Gaon understands the Yerushalmi.)