One well known problem with dates provided by personal recollection is that the degree of accuracy may vary considerably. This is particularly true where the people involved do not use calendrical dates on an everyday basis. Information on more recent events is better remembered than for those long past. Some people may remember their date of birth to the nearest day, others may know the year and some will only be able to locate an event relative to another event or within a broad time span. The concept of levels of accuracy in specifying time is known as granularity.

In the Mambila Population Database, several ways of specifying dates are used. Each set of temporal information has a 'period' field where relative dates can be specified.

Where some children in a birth list have a full date or year or birth but others do not, it is often possible to roughly estimate the missing dates through birth sequence. If child A was born in 1953 and child C in 1955, the intervening birth probably took place in 1954.

Through birth lists, mission registers and birth certificates a framework of fairly accurate dates can be obtained. With register and certificate data, the closer the date of registration is to the date of birth the more accuracy is likely.

By recording relative dates and periods, it is easier to match people accurately where names may be common and grandchildren may bear their grandparents names. It is also easier to link people from census surveys to more specific data in registers etc.