I resolved importing error first examining the rss.php import script and the format used in WordPress to store post date in MySQL.
Then I used in this workaround:
1) I used Find and Replace in a text editor to replace every <pubDate> and </pubdate> entry in the Thingamablog’s xml export file with “*”
2) I used Find and Replace to change every <dc:date> and </dc:date> entry into <pubDate> </pubDate>
3) Finally I used the Import RSS tool in WordPress to load the modified xml export file
All posts, category and post’s date are correct now!
Bye
Matteo
