At once

Sounds like time to me.

It's not an adverb, but an idiomatic preposition phrase. "Once" does not have the same meaning as when it is used without the preceding "at". "At" and "once" are inseparable for this sense and are probably best analysed as a compound preposition.

The PP functions as a temporal adjunct in examples like yours.

I would like to comment on the form of this structure (not the function, as it is clearly a temporal element). This is a (semi-)idiomatic/fossilised prepositional phrase (i. e. not a preposition per se), as it consists of a preposition (at) as head and something that was originally an NP as complement. If you look at it diachronically, it meant 'in one company'.