fastest way to find difference values, within a certain range, between each element in two large vectors. Too large for bsxfun alone

current having to use two loops but it is not optimized for time. I want to do this in the fastest way possible because arrays can be quite large (12mil-6mil elements, in ascending order)

here is the code i have so far. the point is to create a histogram of differences from tmin to tmax (for example -100000 to 100000) with 1000 as the binsize. c is the counts in the histogram. HELP!!

function [c, dt, dtEdges] = coincidence4(time_a,time_b,tmin,tmax,binsize)

% round tmin, tmax to a intiger multiple of binsize:
if mod(tmin,binsize)~=0
    tmin=tmin-mod(tmin,binsize)+binsize;
end
if mod(tmax,binsize)~=0
    tmax=tmax-mod(tmax,binsize);
end

dt = tmin:binsize:tmax;
dtEdges = [dt(1)-binsize/2,dt+binsize/2];

c = zeros(1,length(dt));

Na = length(time_a);
Nb = length(time_b);

tic1=tic;
tic2=tic1;

bbMin=1;
for aa = 1:Na
    ta = time_a(aa);
    bb = bbMin;

      while (bb<=Nb)
          tb = time_b(bb);
          d = tb - ta;
          if d < tmin
              bbMin = bb;
              bb = bb+1;
          elseif d > tmax
              bb = Nb+1;
          else

              index = floor((d-dtEdges(1))/(dtEdges(end)-dtEdges(1))*(length(dtEdges)-1)+1);

              c(index)=c(index)+1;
              bb = bb+1;
          end
      end

end

toc(tic1)
end