Shift time series to start from zero H:M:S:MS (possibly in Matlab) - matlab

Question

I have some ECG data for a number of subjects. For each subject, I can export an excel file with the RR interval, Heart Rate and other measures. The problem is that I have a timestamp starting at the time of recording (in this case 11:22:3:00).
I need to compare the date with other subjects and I want to automate the procedure in Matlab.
I need to flexibly compare, for instance, the first 3 minutes of subjects in condition 1 with those of sbj in condition 2. Or minutes 4 to 8 of condition 1 and 2 and so forth. To do this, I am thinking that the best way is to shift the time vector for each subject so that it starts from 0.
There are a couple of problems to note: I CANNOT create just one vector for all subjects. This would be inaccurate because the heart measures are variable for each individual.
So, IN SHORT I need to shift the time vector for each participant so that it starts at 0 and increases exactly like the original one. So, in this example:
H: M: S: MS RR HR
11:22:03:000 0.809 74.1
11:22:03:092 0.803 74.7
11:22:03:895 0.768 78.1
11:22:04:663 0.732 81.9
11:22:05:395 0.715 83.9
11:22:06:110 0.693 86.5
11:22:06:803 0.705 85.1
11:22:07:508 0.706 84.9
11:22:08:214 0.749 80.1
11:22:08:963 0.762 78.7
11:22:09:725 0.766 78.3
would become:
00:00:00:0000
00:00:00:092
00:00:00:895
00:00:01:663
and so forth...
I would like to do it in Matlab...
P.S.
I was working around the idea of extracting the info in 4 different variables.
Then, I could subtract the values for each cell from the first cell.
For instance:
11-11 = 0; 22-22=0; 03-03=0; ms: keep the same value
Maybe this could kind of work, except that it wouldn't if I have a subject that started, say, at 11:55:05:00
Thank you all for any help.
Gluce

Answer 1

Basic timestamp normalization just subtracts the minimum (or first, assuming they're properly ordered) time from the rest.
With MATLAB's datetime object, this is just subtraction, which yields a duration object:
ts = ["11:22:03:000", "11:22:03:092", "11:22:03:895", "11:22:04:663"];
% Convert to datetime & normalize
t = datetime(ts, 'InputFormat', 'HH:mm:ss:SSS');
t.Format = 'HH:mm:ss:SSS';
nt = t - t(1);
% Reformat & display
nt.Format = 'hh:mm:ss.SSS';
Which returns:
>> nt
nt =
1×4 duration array
00:00:00.000 00:00:00.092 00:00:00.895 00:00:01.663
Alternatively, you can normalize the datetime array itself:
ts = ["11:22:03:000", "11:22:03:092", "11:22:03:895", "11:22:04:663"];
t = datetime(ts, 'InputFormat', 'HH:mm:ss:SSS');
t.Format = 'HH:mm:ss:SSS';
[h, m, s] = hms(t);
[t.Hour, t.Minute, t.Second] = deal(h - h(1), m - m(1), s - s(1));
Which returns the same:
>> t
t =
1×4 datetime array
00:00:00:000 00:00:00:092 00:00:00:895 00:00:01:663