Export of nodes sets created dates to Null

I was having trouble importing the created date into my 6.x site from my 5.x site. After checking node table in the 6.x database, I found that the "changed" field had the correct value, but the "created" field listed the date and time of when it was imported.

I then checked the same values in the export code generated by my 5.x site (node export module 5.x-2.3). In the export code, the "created" value for each node is "Null" ("created' => NULL" ) but each changed field has a value. It doesn't seem to matter which node I try to export--created remains "NULL" for each.

I'm not sure what might be causing this or how to test further. I'm also not sure if this is a bug or not, so I have not labeled it as such.

Any help would be appreciated.

Thanks,

JV