I think you might be looking fot xtick.
otherwise, please make a mockup picture of how it should look.
T=[]; B=[]; xt = []; xtl = {};
for day=1:29
xt(end+1) = numel(T)+1;
xtl{end+1} = num2str(day);
t1=day+(10+([1:1400]/60/60))/24;
t2=day+(16+([1:1520]/60/60))/24;
bmp1=(1+rand(size(t1)))*10+80;
bmp2=(1+rand(size(t2)))*10+83;
T=[T;t1';t2'];
B=[B;bmp1';bmp2'];
endfor;
plot(B);
skip=floor(numel(xt)/5); % prevent too many labels
set(gca, 'xtick', xt(1:skip:end), 'xticklabel', xtl(1:skip:end));