Ømålsordbogen (the Dictionary of the Insular Dialects) describes the Danish dialects on Sealand, Funen and surrounding islands. It covers a period from 1750 to 1945, 1850-1920 being its core period (the majority of the informants were born between 1840 and 1910). As such it can clearly be labelled an historical dictionary. But at the outset, during the first decades of the 20^th century, the intention was to make a contemporary synchronic dictionary, partly inspired by early structuralism and based on primary sources and newly collected data. However, exact chronological delimitation of possible sound changes and changes in vocabulary before 1850 was difficult already then. Furthermore, constant delays of the editing process (vol. 1 appeared only in 1992) and very rapid linguistic changes soon made the original plans of a contemporary dictionary impossible. Thus, traits of diachrony and historical perspective, especially within vocabulary and ethnological descriptions, but at times also within indications of pronunciation, have been introduced into a dictionary that in principle is still trying to fulfil its original synchronic ideal. The case of Ømålsordbogen draws attention to the fact that in order to make consistent decisions on lexicographic markings of chronology, it is necessary to take the sources drawn upon into account and to have an idea of language change both theoretically and empirically with regard to the language(s) treated.