sortしてrenameするだけ

gistfile1.rb

#!/usr/bin/ruby -Ks

# -Ks:convert code to Shift_JIS

dirpath = ARGV[0] # directory path XXX/

def rename_files(_dirpath)
    fnames = Dir.glob _dirpath+'**'
    fnames_created = fnames.map { |fname|
        [fname, File.mtime(fname)]
    }
    ascending_fnames = fnames_created.sort {|a,b|
        a[1]<=>b[1]
    }.map{ |sorted_fname_created|
        sorted_fname_created[0]
    }

    p ascending_fnames

    idx = 0
    ascending_fnames.each do |fname|
        puts fname
        idx_str = idx<10 ? '0'+idx.to_s : idx.to_s
        File.rename(fname, _dirpath+'/'+idx_str+File.extname(fname))
        puts _dirpath+'/'+idx_str+File.extname(fname)
        idx += 1
    end
end

def test_rename_files
    system("mkdir TESTDIR")
    for alphabet in ('a'..'z')
        system("touch ./TESTDIR/#{alphabet}.png")
        sleep(0.1)
    end
    rename_files('TESTDIR/')
    system("rm -rf ./TESTDIR")
end

#test_rename_files
rename_files(dirpath)