Library catalogues are a great resource for metadata. Yet, in many cases early printed books were collated differently, even within the same library. This is largely the result of changing bibliographical practices in the past decades in which only slowly a standard emerged. Therefore, we often find metadata that is not standardised.The date of publication is often far from straight-forward. It may just be an estimation with words like ‘ca.’, ‘um’ or ‘etwa’; it may include two years, such as 1549/50; or it may be displayed in Roman numerals, which sometimes differ from modern-day practice, such as ‘MDXXXX’.