Confidence intervals for difference between change scores

#1
Hi All,

I have two groups tested at four timepoints with a survey of ability. I have compared the groups for change from baseline at each timepoint and have confidence intervals for each change score. Now i want to compare the change scores between each group. Obviously subtracting change scores is not a problem but how would i get a confidence interval for the difference between change scores?

Im using R,

Appreciate any help,

R