Perhaps a bit convoluted, but this appears to work. I haven't compared it extensively to the orginal data. Note that the display format only modifies how the data appears. The time is still there.
data = readtable('Book1.xlsx',"TextType","string");
% your Julian_dates are surrounded by quotes. Remove them
data.Julian_Time = erase(data.Julian_Time,"'");
% use datetime to convert the combined date string to a datetime.
% DDD indicates day-of-year
data.Julian_Time = datetime(data.Year+"-"+data.Julian_Time,'InputFormat','uuuu-DDD-HH:mm:ss.SSS');
data.Julian_Time.Format = 'MM/dd/yyyy'
