Board Thread:The Reference Desk/@comment-71.227.142.141-20121227101010/@comment-5617235-20121227191558

Well, let's see. The Christmas special takes place in 1892 and Human Nature/The Family of Blood in 1913, so that's a 21-year difference between them. Timothy is obviously younger than 21 in HN/TFoB, so theoretically, Captain Latimer could have remarried a few years after The Snowmen and had a third child, Timothy.

Or maybe Timothy is Captain Latimer's nephew, or third cousin once removed, or some other relation. Whatever you think happens, it's possible!