A zero in UNIX time is January 1st, 1970, so that will not work.
Instead, try a custom date format using an ICU format. Then you can enter a BC date directly using the format string ‘G’ to represent the era (AD or BC).
If you want to see the equivalent time in seconds use a function column, but for plotting use the Date column so you get the time line correct in the graph.
Here is an example:
See the ICU Users guide for all the formatting options.
Does this work for your case?
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This reply was modified 4 years, 3 months ago by dgteam. Reason: Clarified the text