ts startdate

Dear R-users,

I am facing a trivial problem when trying to parameterise the start 
date of a time series object. I am working with monthly data (104) 
performing n-steps-ahead (6) forecasts and using a fixed window size 
(36). At the end of calculations I have a list  that contains 69 
forecasts. I have no problems in fixing the window size by 
parametrization, e.g.

k<- control variable in a for loop
my.length<-36
past<-window(X,start=c(1980,k - my.length + 1),end=c(1980,k))

The integer defining the month number can be arbitrary set to: 6,16,46 
etc and it always works. This, however, does not work when I try to 
parameterise the start date of the result list

my.forecasts<-unlist(my.result.list)
my.forecast.results<-ts(my.forecasts,start=c(1980,my.length+6),frequenc

=12)

The message I recive is:

Error in ts(my.frecasts, start = c(1980, my.length + 6), frequency = 
12)
: 
       invalid start

It seems that it works only for month integer up to 12. Does someone 
know how to overcome this problem? Many thanks in advance.

I am using R 1.5.1 on Windows 2000




---
Gregor Gawron