Assuming you've already isolated the datetime strings,
% Convert string to datetime
dtstr = '2018:335:00:00:11';
dt = datetime(dtstr,'InputFormat', 'u:D:HH:mm:ss');
% if you want that in serial date format
n = datenum(dt);
If you need help isolating the strings, leave a comment (hint: regexp).
Note that the string format uses "D" for day-of-year and "u" for ISO-year in place of the Gregorian year (y).
