fit does not accept same range parameters as plot for time data

[<pl...@pi...>]

Hi

I have subset of a datafile that I can plot with plot command by 
defining a range.
The same range parameter is refused by fit:

     f(x)=a*exp(-(x-t0)/b)+c
     format="%Y%m%d %H %M"
     set timefmt format
     set xdata time
     plot ["20150925 02 33": "20150925 07 14"] "valana.txt"  u 1:4 w l

     a=b=c=t0=1;   fit f(x)  ["20150925 02 33": "20150925 07 14"] 
"data.txt"  u 1:4 via a,b,c


data looks like this:

20150923 15 20 22.3 16.1
20150923 15 40 22.3 16.1
20150923 15 49 22.9 16.4

The fit command shows the following error:

   internal error: substring range specifiers must have integer values


I thought the intention was that the two commands functioned in a 
similar way. Why can't fit accept the range specifier in the same format 
as plot?

Regards, Peter.