Are iSeries AS/400 (DB2?), Lotus 1-2-3, and Excel date/time formats the same?
No.

Db2 for i (aka AS/400) dates are stored internally as a Scaliger number (aka Julian Day number)

The number of days since noon on Monday, January 1, 4713 BC.

select hex(current_date)
from sysibm.sysdummy1


returns 002583EC as of about 3pm MDT April 30,2019

The decimal value is 2,458,604

Which agrees which the current value on the web page linked to above