Often year of birth is calculated by subtracting the age given in a census from the census year.

Apart from 1841, the census was taken at the end of March/beginning of April and was supposed to record the age at last birthday.

This would mean that approximately 75% of the population would not have had their birthday for the census year.

Maybe we should get into the habit of quoting birth years obtained from census data in a format similar to old style/new style dates.