From source: “One blustery hearths warming eve, Derpy told the Doctor that if he lived a hundred years, then she would be there a hundred plus a day to be sure he was never lonely. The doctor, knowing that the most she could possibly hope to see was sixty hearth warmings (even if she stopped eating so many muffins), challenged her by asking what she would do if he lived to be one hundred plus two days. She thought for several minutes, and decided that if this were so then she would just have to live a hundred plus three days. Knowing she was cornered, and a little sad as well, he explained to her that he has already lived far longer than that, and would live longer still. She asked how old he was, and he answered. Both were quiet. Derpy spoke first, with a crack in her voice, and asked him to wait a little bit, so that she might be able to catch up to him someday. He smiled and promised he would wait, though if he waits too long she might grow older than him, he warned. She was left speechless with worry, and aged very carefully since then.”