How can I go backward in time using datenum? - matlab

Question

I have these two dates.
startTime = '4/2/2004 12:45'
endTime = '4/3/2004 18:15'
I want to find the time that is exactly 1.5*(startTime-endTime) in the past. And in the format I have rather than datenum. The time progresses in 5 minute steps in my dataset.
I am doing this, but not sure where to go from where:
startTime = datenum('4/2/2004 12:45');
endTime = datenum('4/3/2004 18:15');
Finally, I do something like this to generate filenames for some batch processing.
for l = 1:timeSteps
precipFileNames{l} = strcat(fileparts(refFile), filesep,'RATE.',datestr(startTime, 'yyyymmdd.hhMMss'), '.tif');
startTime = addtodate(startTime, 5, 'minute');
end

Answer 1

You were close. You can use at least two different approaches, datetime or datenum.
Using datenum:
startTime = datenum('4/2/2004 12:45');
endTime = datenum('4/3/2004 18:15');
anotherTime = now;
inPast = anotherTime + (startTime - endTime) * 1.5;
datestr(inPast, 'yyyymmdd.hhMMss');
Or, using datetime:
startTime = datetime('4/2/2004 12:45', 'Format', 'MM/dd/uuuu HH:mm')
endTime = datetime('4/3/2004 18:15', 'Format', 'MM/dd/uuuu HH:mm')
anotherTime = datetime;
inPast = anotherTime + (startTime - endTime) * 1.5;
datestr(inPast, 'yyyymmdd.hhMMss');
startTime = startTime + minutes(5);