The problem is that 25 of ‘data_datetime’ is NaT (not a time, analogous to NaN, not a number). The datenum function cannot convert NaT to anything.
D = load('datetimes.mat');
dt = D.data_datetime;
natv = nnz(isnat(dt))
produces:
natv =
25
One option is to simply interpolate the times:
dtfm = fillmissing(dt,'linear');
s = datestr(datenum(dtfm));
That runs without error.