SDID with time fixed effect and state fixed effect

[Wenhan Yan]

I am new to SDID, and my first question is I should add time fixed effect and state fixed effectwhen I perform SDID in Stata (i.e. i.year and i.fips), but when I add those two into covariates, I received error code:

factor-variable and time-series operators not allowed

sdid r_wo state date treat, vce(placebo) covariates(pop_1000 r_black r_notwhite unemploymentrate income_per_capita r_HS r_college covid lgdp, projected) seed(1234) graph

and produce following graph:

and the effect is:

Synthetic Difference-in-Differences Estimator

-----------------------------------------------------------------------------
        r_wo |     ATT     Std. Err.     t      P>|t|    [95% Conf. Interval]
-------------+---------------------------------------------------------------
       treat |  -0.02918    0.01323    -2.20    0.027    -0.05511    -0.00324
-----------------------------------------------------------------------------
95% CIs and p-values are based on Large-Sample approximations.
Refer to Arkhangelsky et al., (2020) for theoretical derivations.

My second question is the green triangle on the graph, are those indicate the year selected for the base year? I am just not sure