Assuming that the file has been opened and all contents read into the matrix dateData (see dlmread for examples on reading numeric data from a text file) then you can do the following
% subset of the data
dateData = [1011949; 2011949; 3011949; 4011949; 5011949; 6011949; 7011949; 8011949];
% determine the number of rows/dates
numDates = size(dateData,1);
% for each date, reproduce it four times for the hours of 0, 6, 12, 18
dateData = cell2mat(arrayfun(@(x)repmat(x,4,1),dateData,'UniformOutput',false));
% now create a column of 0,6,12,18 repeated numDates times
hourData = [0; 6; 12; 18];
hourData = repmat(hourData,numDates,1);
% "attach" the hours to the dates
newData = [dateData hourData];
In re-constructing dateData we use arrayfun in order to apply a function to each element of the input array (so to each date). The function is repmat which allows each date to be replicated from a single element into a 4x1 array. The output from each date is a 4x1 array, each as an element in a single cell array. We then convert this cell array to a matrix using cell2mat.
We do something similar to hourData - we just replicate the 4x1 array numDates times for each of our dates. We then "add" the two columns together to create the final matrix.
Try the above and see what happens!
