You won't be able to do this with tables. Using Accessing Data in Tables, extract the data, take the diff along dimension 2, then build a new table from the results.
Date1 = [1:4]';
Date2 = [6 8 9 11]';
Date3 = [10 11 14 11]';
User = ["A" "B" "C" "D"]';
T = table(User,Date1,Date2,Date3)
T2 = diff(T{:,2:end},[],2)
T2 = table(T.User,T2);
T2=splitvars(T2);
T2.Properties.VariableNames = T.Properties.VariableNames([1,3:end])
