Perceptions of temporality have a significant impact on the legitimacy of a nascent field. Expectations play a particular role in legitimation in such contexts, and research identifies other types of temporal perceptions and dynamics that relate to change and the persistence of institutions. However, a coherent understanding of how temporality and legitimacy are intertwined in the development of nascent fields is still lacking. Drawing on a grounded study of the emergence of a solar energy field in a northern country where legitimation has been particularly challenging, we develop a framework for sustained temporal illegitimacy in nascent fields. We find that legitimation in temporal terms is apparent in their momentum, and that illegitimacy is produced by two types of asynchronicity that reduce momentum – asynchronicity in the perceived rhythm of local field development as reflected against the rhythm of an acute global challenge, and asynchronicity in the timing of engagement among participating communities. These two types produce a lack of concerted action and stalling momentum. We discuss the implications of our findings for studies on temporality and institutions in the legitimation of nascent fields, as well as for related research on expectations and hypes related to the emergence of a new field.