The length of a life

Many of the inscriptions in the catacombs and cemetaries of the first Christians in Rome relate the exact number of years, months, and days which a person lived, and many of them also specifiy the day on which they died, but none mention the person's birthday.  Clearly they knew they knew the 'real' birthday, but they simply do not mention it.  However, the inscriptions do frequently speak of the day on which they died as the day on which they were "born in peace", or something similar.