Learn Croatian: Istog mjeseca došlo je do razorne erupcije. - The same month, a devastating eruption occurred.

This is the bare genitive of time, which is the focus of this lesson. When a time word like mjesec (month), dan (day), or godina (year) is modified by an adjective or a pronoun like isti (same), it takes the genitive case to indicate when something happened. No preposition is needed.

The translation says 'an eruption occurred', but the Croatian uses došlo je do. How does this construction work?

Došlo je do is a very common impersonal expression meaning 'there occurred' or 'it came to'. It is built with the neuter past tense of doći (to come) plus the preposition do. Croatians use this phrase constantly for sudden events, accidents, or developments, rather than using a direct verb for 'happen' or 'occur'.

Is razorne erupcije in the genitive case because it's also part of a time expression?

No, razorne erupcije is in the genitive case simply because it follows the preposition do from the phrase došlo je do. The preposition do always forces the following noun phrase into the genitive.