I am a Stata 15.1 user.

Before posting my question here, I searched in the forum but could not find any proper answer.

I am doing an analysis of a strategy implemented by a four-year-old corporate company and this implementation was given in different cities which all of its employees were received the treatment or not in 2016-2019. The strategy change date is 2017.

I apply the quasi-experimental method (difference-in-differences) and I am concerned about the validity of my results. Therefore, for the validity check of my difference-in-differences analysis, I am planning to do placebo tests with fake treatment dates before and after the actual date of treatment. However, due to the company was established in 2016, there is no data before 2016. Although I can use "2018" as a fake treatment year, I am wondering how can I do a placebo test before the actual date with a single period (just 2016) data and do I need to use any fake treatment year before the actual date? Is it enough to use only 2018 as a fake treatment date for placebo test to check the validity of the actual treatment date (2017) results?

Any alternative suggestions for the difference in differences placebo test or robustness of the difference-in-differences test results would be much appreciated.