How to prevent fix() from converting Dates into numeric

Dear list members

Here is a strange problem that I have had for a long time, without 
finding out how to solve it. Whenever I use fix() on a data.frame that 
contains Dates, these are converted to numerics. As shown by the very 
simple example:

a <- data.frame(var1 = 1, today = Sys.Date() )
a
fix(a)
a

Why is that? And can anything be done against it?

Many thanks for your help!

Best,

Christian Gold
www.uib.no/people/cgo022