The General Register Office (GRO) indexes births, marriages, and deaths by the quarter in which the event was registered: M (March), J (June), S (September), D (December). Often these are the most precise dates available when the original certificate has not been obtained.
Enter the quarter letter followed by the year — for example:
M1852
This means the event was registered in the March quarter of 1852. Note that this does not mean the event itself occurred in that quarter — births could be registered up to six weeks after the event, so a birth registered in M1852 might have taken place between mid-November 1851 and late March 1852.
In addition to the quarter letters, the prefixes "early", "mid", and "late" are also supported for less precise estimates.
Note — GRO quarter dates are not supported in the GEDCOM standard, so there is a loss of precision when exporting.