Time axis problem

["Barry B. Baker" <bbaker@xxxxxxx>]

Hi all,
 
I'm rather new to Ferret and have a problem that I'm sure is simple.  Could not find a solution in the FAQ or the thread that exactly fit.
 
I am trying to subtract the following
 
set data historic  (tmax i=1:84 j= 1:36, l = 1:12, t= 16-jan:16-dec)
set data future (tmax i=1:84, j=1:36, l=1225:1236, t=31-jan-2099:31-dec-2099)
 
let tmaxdiff = tmax[d=2] - tmax[d=1]
 
The error message is: Ambigous coordinates on T axis
 
I tried the following without success
let tmaxdiff = tmax[d=2, gt=tcltmax[d=1]@asn] - tmax[d=1]
 
Any help would be greatly appreciated.
 
Cheers,
Barry