For surviving children born in the three to five years before the survey, HWDAYOFBIRTH (HW16) reports the day of the month on which the child was born.
The DHS recode manuals for Phase 2 forward report that the day of birth of the child
is used in conjunction with the date of measurement of the child to more finely calculate the age of the child in days when computing anthropometric measures. This level of accuracy can be important in very young children. The age of the child calculated from the day of birth information is not used as a background characteristic for tabulations, but purely to produce a more accurate set of anthropometric measures.
DHS analysis thus uses HWDAYOFBIRTH and the date of the child's height and weight measurement to precisely calculate child malnutrition indicators such as height-for-age (stunting) and weight-for-age (underweight) Z-scores.