subtracting time - datetime

Hi All, I'm struggling to substract the initial time from a datetime variable and get the results including milliseconds. I'n doing as below but I get "Error using datetime (line 668), Input data must be a numeric array, a string array, a cell array containing character vectors, or a char matrix.

t1 = datetime(t-t(1),'Format','hh:mm:ss.SSS');

Can you help me to figure out what's wrong?

Cheers

[dpb]

Is t already a datetime variable? If so, then simply

t1=t-t(1);  % t1 will now be a duration, NOT a datetime
t1.Format='hh:mm:ss.SSS';

If t it isn't a datetime, then need to know what it is; if it is a recognizable string format, then you've got to first create the datetime variable before you can subtract the first element --

t1=datetime(t,'Format','hh:mm:ss.SSS');     % create datetime -- t must be standard time format or will need 'InputFormat', too
dt=t1-t1(1);                                % now create the duration time variable

[Luis Eduardo Cofré Lizama]

That was quick and effective!, t was already a datetime. Thansk works!