Without a documented date on a vital record, the date should be documented as about/uncertain. The fact that you only have a year suggests that you have an oral history, census estimated date, estimated from a death record, tombstone estimate based on age at time of death or some such. Oral history such as auntie Em says yup I'm sure of the date. That's the same year Dorothy got sucked up in the tornado. That doesn't fly. You should only use certain/exact when you are absolutely sure of the year.