Can I replace this nested for loop with a faster process?

I have a variable, A, which is a Nx1 cell array, where N is a variable number which can vary between 1 and 1000. Each row within A contains another large(ish) cellarray, which can vary from anywhere between 100 and over 1000 rows. Example:

>> A

A = 

    {583x20 cell}
    {371x20 cell}
    {626x20 cell}
    {450x20 cell}

Column 19 within each row of A contains a set of times which have been converted using datenum, e.g.

>> [A{1,1}{1:5,19}]'

ans =

          735965.791900579
          735965.791946875
          735965.791993171
          735965.792039468
          735965.792085764

I have written the following code to examine each row of column 19, within each row of matrix A, and make a comparison against some existing values, which uses a nested for loop. I think this is probably quite inefficient, especially if matrix A contains 1000 rows. How can I optimise this code, or replace it with several functions to speed this process up?

metdata_boundaries = datenum({'07:37:30' '10:40:29.99'...
                              '10:40:30' '13:43:29.99'...
                              '13:43:30' '16:46:29.99'...
                              '16:46:30' '19:49:29.99'...
                              '19:49:30' '22:52:29.99'});

for i = 1:size(A,1)

      A{i,1}(:,19) = num2cell(datenum(A{i,1}(:,13))); % Column 13 contains times in the format 'HH:MM:SS.MS'

      for x = 1:size(A{i,1},1)

          if A{i,1}{x,19} >= metdata_boundaries(1) && A{i,1}{x,19} <= metdata_boundaries(2)

              A{i,1}{x,20} = 'Use 09:09 met data';
              met0909 = 'True';

          elseif A{i,1}{x,19} >= metdata_boundaries(3) && A{i,1}{x,19} <= metdata_boundaries(4)

              A{i,1}{x,20} = 'Use 12:12 met data';
              met1212 = 'True';

          elseif A{i,1}{x,19} >= metdata_boundaries(5) && A{i,1}{x,19} <= metdata_boundaries(6)

              A{i,1}{x,20} = 'Use 15:15 met data';
              met1515 = 'True';

          elseif A{i,1}{x,19} >= metdata_boundaries(7) && A{i,1}{x,19} <= metdata_boundaries(8)

              A{i,1}{x,20} = 'Use 18:18 met data';
              met1818 = 'True';

          elseif A{i,1}{x,19} >= metdata_boundaries(9) && A{i,1}{x,19} <= metdata_boundaries(10)

              A{i,1}{x,20} = 'Use 21:21 met data';
              met2121 = 'True';

          else

              A{i,1}{x,20} = 'No met data available';

          end

      end

end

if exist('met0909','var')
    load('metdata_0909.mat')
end
if exist('met1212','var')
    load('metdata_1212.mat')
end
if exist('met1515','var')
    load('metdata_1515.mat')
end
if exist('met1818','var')
    load('metdata_1818.mat')
end
if exist('met2121','var')
    load('metdata_2121.mat')
end