Date arithmetic in Leopard

[clarifix]

In http://developer.apple.com/documentation/applescript/conceptual/applescriptlangguide/AppleScript.ab.html

Apple documents this:

AppleScript supports these operations with the + and - operators on date and time difference values:

date + timeDifference
–result: date date - date
–result: timeDifference date - timeDifference
–result: date
where date is a date value and timeDifference is an integer value specifying a time difference in seconds.

Look at this code:

set theDate to date “1-1-1904 12:00:00”
set DateDiff to ((current date) - theDate)
set testDate to theDate + DateDiff

In Tiger, the result is today’s date, in Leopard, you get a date somewhere in february 1907.
I tried figuring out where thing go wrong:

(current date) + 5.3687E+8

is still OK

(current date) + 5.36871E+8

returns a wrong date

The workaround for my problem above was to cut the number in chunks of 5.3687E+8, and add them in a loop, adding the remainder of the division after the loop.
Am I misreading Apple’s documentation ( again ) or did I stumble over a bug?