Unlike the usual census return, which took the age, and from that estimated the year of birth, the 1939 register gave the actual date of birth - day, month, year.
Often, however, as noted, the year does not align with the birth registration, mostly being one year off, sometimes more.
(Example, someone's birth was registered in the 1st Quarter of 1897, and the 1939 register gives the date of birth as 12 Feb 1898. A year difference.)