Ambiguous coordinates on time axis

[David Muhati <muhati@xxxxxxxxxxxxxxxxx>]

Hi all,
Came across this error message while calculating the difference between 
myvar (variable) for 2 different periods for 1 year and 4years 
respectively. Not sure what the best approach is of this two; NB: d=1 is 
the 1year period e.g 1997 and d=2 is the 4 year (1995 to 1998)
Approach 1
yes? fill myvar[d=1,l=@ave]- myvar[d=2,l=@ave]
yes? go land
error: ambiguous coordinates on time axis
Approach 2
yes? fill myvar[d=1,t="1-jan-1997":"31-dec-1997"@ave]- 
myvar[d=2,t="1-jan-1995":"31-dec-1998"@ave]
yes? go land
Doesn't give an error message.

So which is the best because the results of the two look different?
David