Question

I have an array of Epoch milliseconds (array of numbers) in Matlab. I would like to convert these into UTC date-time format, such as DD-MM-YYYY HH:MM.

Is there a pre-defined Matlab way to do this or will I have to write my own function?

Answer 1

Suppose, you start with a vector time_unix, then:

>> time_unix = 1339116554872; % example time
>> time_reference = datenum('1970', 'yyyy'); 
>> time_matlab = time_reference + time_unix / 8.64e7;
>> time_matlab_string = datestr(time_matlab, 'yyyymmdd HH:MM:SS.FFF')

    time_matlab_string =

    20120608 00:49:14.872

Notes:

1) See the definition of matlab’s time.

2) 8.64e7 is number of milliseconds in a day.

3) Matlab does not apply any time-zone shifts, so the result is the same UTC time.

4) Example for backward transformation:

>> matlab_time = now;
>> unix_time = round(8.64e7 * (matlab_time - datenum('1970', 'yyyy')))

unix_time =

             1339118367664

To summarize, here are two functions:

function tm = unix2matlab(tu)
    tm = datenum('1970', 'yyyy') + tu / 864e5;
end
function tu = matlab2unix(tm)
    tu = round(864e5 * (tm - datenum('1970', 'yyyy')));
end

The matlab time here is numeric. You can always convert it to string using datestr()

Update for nanoseconds

time_unix_nanos = 1339116554872666666;
millis = round(time_unix_nanos / 1e6);
nanos = time_unix_nanos - 1e6 * millis;
time_matlab = unix2matlab(millis);
s = [datestr(time_matlab, 'yyyymmdd HH:MM:SS.FFF'), num2str(nanos)];

        s =
        20120608 00:49:14.872666666