[Gnuplot-info] problem with 'fit' when xdata set to date/time

[DU <des...@gm...>]

I'm not subscribed to the list, so please cc me directly.

I have attached a file named "test.dat" containing sample data.  One
column is a date, the other column is a value.  I want to plot these
values and also a linear fit to the data.  To do this, I created
"test.plt", also attached.

When I ran this with a copy of test.dat where the date field was a
simple number, I got the expected result (a negatively sloped line). 
When I use the date field, I get a roughly horizontal line, even
though I have used the "set xdata time" and set the timefmt.  I get
the same outcome with both gnuplot 3.7 and 4.0.

All the options and commands I'm using are in the file, nothing else
is on the command line.

I must be doing something wrong--but what?