In one-year ACS, why do ALL infants (age<1) have birth year same as the ACS survey year?

I am currently using ACS one-year surveys.

When I check the birth year of the infants (age<1) in a given ACS survey, their birth years are equal to the survey year for ALL infants. If I understand correctly, ACS survey is implemented every month throughout the year. Then, for example, if a household was surveyed in March of 2004 for 2004 ACS and if they have a 6-months-old infant, the birth year of this infant has to be 2003. But why do birth years of ALL infants same as the ACS year?

As noted on the comparability tab for BIRTHYR, self-reported birth year is available only for the 1 percent samples for 1900 and 1910, as well as the 1900 100% database. In all other samples, self-reported birth year is not available but has been calculated by subtracting AGE from the survey year (MULTYEAR in the multi-year ACS/PRCS samples; YEAR in all other samples).