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

1

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?

2

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).