In the UK, since 1837, births, marriages and deaths should have been registered with the local registrar. That results in a certificate recorded in the local register. We can pay to obtain a copy of a particular certificate from a register. However, the indexes to the register are freely available, and list the names of everyone registered in a particular calendar quarter.

Births were required to be registered within 6 weeks, 42 days, of the event.

Q1 1892 ran from 1 January 1892 to 31 March 1892. Any birth registered between those dates will be listed in the index for that quarter. The earliest birthdate for the baby (assuming the rules have been obeyed) is thus 42 days before the beginning of the quarter: 20 November 1891. The latest birthdate is the very end of the quarter, 31 March 1892.

The birthdate calculator applies this 6-week rule and gives earliest and latest birthdates for a particular quarter.