Are iSeries AS/400 (DB2?), Lotus 1-2-3, and Excel date/time formats the same? Are the formats for dates/times from AS/400, to Lotus 1-2-3 that carried over into excel...with the date being the number of days since 1900-1-1 when mistaking 1900 for a leap year as an integer, and the decimal portion being the time.

Are they all the same format?
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