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
