If you have a newer version of Matlab, you can parse this directly to a datetime object
a = '2015-033T18:00:00';
d = datetime(a, 'InputFormat', 'yyyy-D''T''HH:mm:ss')
And if you need a datenumber instead:
dn = datenum(d)
For Matlab releases before 2014b, I think you'll have to parse manually:
tmp = textscan(a, '%f-%fT%f:%f:%f')
dn = datenum(tmp{1},1,1) + (tmp{2}-1) + tmp{3}/24 + tmp{4}/(24*60) + tmp{5}/(24*60*60)
