One Month Old

Tristan is one month old! Actually he is four weeks old. Since he was born on the 31st of Jan, when exactly is he one month? Is it the last day of Feb? Is it 3 days after the 28th, Mar 3rd, which is when Feb 31st would be if Feb had that many days? I don't know, but he's growing fast. He now weighs in at 10 lbs. 7 oz. and is 22 inches long.