The General Register Office have used a long established system of indexing dates of birth, marriage and deaths by the quarters in which the event was registered: M (March), J (June), S (September) D (December). Often, these are the most precise date we have for an event, if the original certificate or extract hasn’t been obtained. The GRO quarters can be entered followed by the year, for example:
M1852
Would infer that the event was registered in the March quarter of 1852. This doesn’t mean that the event took place in that quarter—as 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. Technical note: this isn’t supported in GEDCOM, so on export to GEDCOM there is a loss in precision.
In addition to M/J/S/D, “early”, “mid” and “late” are also supported.