HEADER-AEDTM is '00000000' when HEADER-OPERA is 'I'

I am trying to find records in 168 that have been inserted within the time period specified on the PNPCE selection screen like this:

if header-endda eq '99991231' and header-aedtm between pn-begda and pn-endda.

However, when the HEADER-OPER is "I" the value of HEADER-AEDTM is always zeros. I'm confused. Please advise.

Thanks.