I don’t know what Greymatter is. Is it listed as one of the supported importers?
Every post has two datetimes associated with it. One is the datetime in local time when the post was made, the other is the datetime in GMT. The reason these are both stored is to prevent DST changes from modifying the dates when the posts were made and such.
Normal viewing shows the normal timestamp.
RSS feeds display the GMT timestamp.
So if your importer only imported to post_date and not to post_date_gmt, then you’d see exactly what you are, in fact, seeing. A blank timestamp of zero would correspond to the zero point of the uniw epoch, or Jan 1st, 1970. The default of -1 for the error from having a null in post_date_gmt would give Dec 31, 1969.
So, how exactly did you import these posts? What steps did you take to import them? The fix is the same either way, correct the timestamps in the post_date_gmt column. But knowing how you imported the posts might help to fix that particular problem, if the devs know where the problem lies.
In the meantime, you can probably change the timestamps on each post to correct the timestamps in the database. Just edit the post and check the “Edit Timestamp” box, then save the post. You don’t have to change the timestamp itself, I think.
