GLLEZL : R12 Journal import from GL_INTERFACE journal header effective date

Hi,

I'm importing transaction lines from GL_INTERFACE table (it was populated by external feeder system). There are many lines with different transaction dates from across the current period. Once import is completed a journal is created with all those lines. The journal header "default effective date" is set to first day of that period (some of the transactions have this particular date but most of them don't). How the journal header effective date is derived? Is there any functionality to amend the logic?

Thanks

Adam