Determining the date of writing is a practical problem often encountered in the study of ancient and medieval texts. The problem is compounded by differences in genre, register and style, but in particular by our frequent dependence on much later copies of lost originals. This article examines how a method of classification with flexible time intervals has been developed for a corpus of medieval Irish annals (c.700–c.1600). It is shown that the method can deal successfully with an unparsed, complex corpus containing contaminated data. A method of extending the model from good quality, uncontaminated data to more complex texts is also demonstrated by identifying temporal characteristics and typical entries in the high-quality source.