string inputStr = @"
1 12 3 17 22 0 0.0
  1.500000000000D+01
 2 12 3 18 4 0 0.0
  6.500000000000D+01
13 12 3 18 12 0 0.0
  4.300000000000D+01
 4 09 12 12 4 0 0.0
  7.400000000000D+01
 5 12 12 3 6 0 0.0
  2.600000000000D+01
 6 12 3 18 11 59 44.0
  0.000000000000D+00
37 12 3 18 2 0 0.0
  4.700000000000D+01
28 12 3 18 4 0 0.0
  4.900000000000D+01
 9 11 9 3 10 0 0.0
  3.600000000000D+01
10 12 3 18 4 0 0.0
  7.200000000000D+01
11 12 3 17 22 0 0.0
  5.700000000000D+01
12 12 3 22 10 0 0.0
  8.000000000000D+01
33 12 3 18 12 0 0.0
  3.200000000000D+01
14 12 3 18 12 0 0.0
  6.300000000000D+01
45 12 3 18 8 0 0.0
  4.300000000000D+01
16 12 3 19 11 59 44.0
  5.000000000000D+00
"
    ;

Regex reg = new Regex(@"(\d{2} \d{1,2} \d{1,2})( \d{1,2} \d{1,2} \d{1,2}\.\d)");
DateTime firstDate = Convert.ToDateTime(reg.Match(inputStr).Groups[1].Value);
DateTime baseDate = new DateTime(2011, 1, 1);
TimeSpan ts = firstDate - baseDate;
string outputStr = reg.Replace(inputStr, new MatchEvaluator((m) =>
{
    return (Convert.ToDateTime(m.Groups[1].Value) - ts).ToString("yy M d") + m.Groups[2].Value;
}));
Console.WriteLine(outputStr);

/*
1 11 1 1 22 0 0.0
  1.500000000000D+01
 2 11 1 2 4 0 0.0
  6.500000000000D+01
13 11 1 2 12 0 0.0
  4.300000000000D+01
 4 08 9 27 4 0 0.0
  7.400000000000D+01
 5 11 9 19 6 0 0.0
  2.600000000000D+01
 6 11 1 2 11 59 44.0
  0.000000000000D+00
37 11 1 2 2 0 0.0
  4.700000000000D+01
28 11 1 2 4 0 0.0
  4.900000000000D+01
 9 10 6 19 10 0 0.0
  3.600000000000D+01
10 11 1 2 4 0 0.0
  7.200000000000D+01
11 11 1 1 22 0 0.0
  5.700000000000D+01
12 11 1 6 10 0 0.0
  8.000000000000D+01
33 11 1 2 12 0 0.0
  3.200000000000D+01
14 11 1 2 12 0 0.0
  6.300000000000D+01
45 11 1 2 8 0 0.0
  4.300000000000D+01
16 11 1 3 11 59 44.0
  5.000000000000D+00
*/