I was able to create a News item with a publication date in the past using this code snippet in Sitefinity 6.2 for Spanish language:
CultureInfo backupCulture = System.Threading.Thread.CurrentThread.CurrentUICulture;
CultureInfo cult =
System.Threading.Thread.CurrentThread.CurrentUICulture = cult;
"Sample item in past"
DateTime(1978, 4, 15))
System.Threading.Thread.CurrentThread.CurrentUICulture = backupCulture;
The item is created as Draft but after publishing its publication date remains the same as set in the code. Here is a reference
The same could be done by using Publishing on a specific date and state a data in the past though the back-end.
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