That fully depends on what the original _time field was based on. That will not be based on your my_unix_time_column , otherwise you could have done a straightforward timechart from the start and didn't have to use this alternative field.
To get an answer to that, you need to look into what was used to originally set _time during the ingestion of this data (some other (possibly incorrectly interpreted) timestamp in the event? current time during ingest? ...). And maybe even need to investigate whether maybe some data source has an incorrect clock setting or so.
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