When dealing with the differential timing DiD setting, we may apply some modern approaches like Callaway, 2020, Borusyak, 2021 (I focus more on the imputation estimator of Borusyak because I am using it)

However, it comes to me a counterintuitive thought that: Why the treatment effect is assumed to start from the event date to the end of the sample period as indicated by (Borusyak, 2021). There should be some confounding events coming and change the pure examined effect. Why the effect do not just stay there just for 2,3,4 years, especially for accounting variables. And the effect for longer time further from the event date should be very messy.

For example, let us say a sample period lasts from 1990 to 2020, and US implement the law in 1993, is it fair to examine the effect of the laws on firms' asset growth by letting the treatment effect staying from 1993 to 2020 ?