Anyone can help me? A Unknown Domain key ERROR Occur in my Timeseries Chart

I am now visualizing my data in the report using Timeseries Chart.
The domain key I am expecting is the "real_t_stamp"as follows:

However, when I switch to preview tab, the timestamp weirdly became “1970-01-01 08:00” and I have no clue about this issue.

It would be high appreciated if any help could be provided!!