How do I make a loop that goes through day by day?

I for that day am doing research and need to create a loop that goes through day by day and also includes the appropriate sunrise and sunset times starting at line "day_2=...". my data looks like; "date/time | data | data | data " where date/time goes in half hour increments and: "date | time_sunrise | time_sunset" where date goes by day.

    %Hudson River Water Quality 1995-2000
    data=xlsread('Hudson_test1.xlsx');
    tm=data(:,1)+datenum(1904,1,1);
    [yr,mo,day,hr,min]=datevec(tm);
    DO_p=data(:,5);
    DO=data(:,6);
    z=data(:,7);

    dif_DO=DO(2:end)-DO(1:end-1);
    air_sea=(1-((DO_p(1:end-1)+DO_p(2:end))/200))*0.5*0.5;
    do_flux=(dif_DO.*z(2:end))-air_sea;

    sun=xlsread('rise_set_test.xls');
    tm_sun=sun(:,1)+datenum(1904,1,1);
    [yr_s,mo_s,day_s]=datevec(tm_sun); 
    rise=(sun(:,2));
    set=sun(:,3);

    %put sunrise and sunset into proper matlab date format
    srh=floor(rise/100);
    srm=rise-100*srh;
    sr=(srh+srm/60)/24;
    sr_tm=sr+tm_sun; %puts in date format

    ssh=floor(set/100);
    ssm=set-100*ssh;
    ss=(ssh+ssm/60)/24 ;
    ss_tm=ss+tm_sun;

    day_2 = tm(2:96);

    j_2 = day_2>=sr_tm(2,1) & day_2<=ss_tm(2,1);
    m_2 = day_2<=sr_tm(2,1) & day_2>=tm(49,1) |day_2>=ss_tm(2,1);
    do_dy_2 = sum(do_flux(j_2));
    do_nt_2 = sum(do_flux(m_2));
    resp_rt_2 = (do_nt_2.*(-1))/(nnz(m_2)/2);
    ttl_resp_2 = resp_rt_2.*24;
    day_resp_2 = resp_rt_2.*(nnz(j_2)/2);
    gp_2 = day_resp_2 + do_dy_2;
    NEM_2 = gp_2-ttl_resp_2;