set_stadata_coords种sta["time"] = sta["time"].astype(np.datetime64)的作用？

[xiangyang1989]

set_stadata_coords种当len==1时的作用？为1为什么要再次做转换？运行astype(np.datetime64)总是报错Casting to unit-less dtype 'datetime64' is not supported. Pass e.g. 'datetime64[ns]' instead.
if len(sta.index) ==1:
sta["time"] = sta["time"].astype(np.datetime64)