mute@ovh:~$ printf '%s\n' 20120728 20120726 20120520 | ./julian.awk
Friday after:
Min: 2012/05/25
Max: 2012/08/03

#!/usr/bin/awk -f

function mk_j(y,m,d,    a, j)
{
        a = int((14 - m) / 12);
        y = y + 4800 - a;
        m = m + 12 * a - 3;
        j = d + int((153*m + 2)/5) + 365*y + int(y/4) - int(y/100) + int(y/400) - 32045
        return j
}

function mk_g(jd,       l,n,i,j,d,m,y)
{
        l = jd + 68569
        n = int(( 4 * l ) / 146097)
        l = l - int(( 146097 * n + 3 ) / 4)
        i = int(4000 * ( l + 1 ) / 1461001)
        l = l - int(( 1461 * i ) / 4) + 31
        j = int(( 80 * l ) / 2447);
        d = l - int(( 2447 * j ) / 80);
        l = int(j / 11);
        m = j + 2 - ( 12 * l );
        y = 100 * ( n - 49 ) + i + l

        return sprintf("%04d/%02d/%02d",y,m,d)
}

{
        y=substr($0,1,4)
        m=substr($0,5,2)
        d=substr($0,7,2)
        dates[++i]=mk_j(y,m,d)
}

function next_fri(jd,   dow) {
        dow = jd % 7
        # friday=4
        return jd + ( (dow > 4) ? 7 : 0 ) + (4 - dow)
}

END {
        min=max=dates[1]
        for (i in dates) {
                if (dates[i] > max) max=dates[i]
                if (dates[i] < min) min=dates[i]
        }

        printf("Friday after:\nMin: %s\nMax: %s\n",
                mk_g(next_fri(min)), mk_g(next_fri(max)));
}

mkdow() {
        local y=$1 m=$2 d=$3
        echo $((
                ((m<3?y--:y-2)+23*m/9+4+d+y/4+y/100*25/4)%7
        ))
}