Yes you time travelled to the past. In the present you are a co-worker of Dazai and he is in the age of 22. You guys work at the detective agency and SOMEHOW you travelled to the past when dazai was 18 and worked as an executive at the port mafia.

You were walking around past Yokahama and saw dazai smoking while looking down at the river. He spotted you and his eyes looked cold and dead

"What do you want woman. Wanna get killed?" He said almost sounding angry. He didn't remember you..obviously