You should name the type timestamp if you want to imply dt + n == dt + n seconds. datetime + n is commonly understood as dt + n *days*.
Is this just an mxDateTime convention, or is it in wider use? (URLs of docs of other languages / libraries would really help to convince me!)
I'm afraid that this really is the de-facto standard.
Then I propose to remove the feature of automatic mixing int/long/float and datetime or timedelta instances in add/subtract operations. You'll have to specify what you want using a timedelta(days, seconds) constructor. That's clearer anyway.
However, lets be clear; is t=t+1 adding 1 (calendar) day or 24 hours (86400 seconds)?
How does it differ? (We already ignore leap seconds.)
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)