Proto-Slavic is the language from which the Slavic language family sprang. It would have been spoken about 500-600 AD. The first written Slavic language is Old Church Slavonic, which appears from 865 AD, and is very close to Proto-Slavic.
Linguistically it is deduced that Proto-Slavic was spoken in the region around the Middle Dnieper. Its ancestor Proto-Balto-Slavic developed long before in the same region, in contact with the Proto-Indo-Iranian language and shares features with it. But that shared dialect continuum broke up about 2500 BC. Proto-Indo-Iranian does not descend from the far younger Proto-Slavic. That makes no sense at all.
On the genetic front R1a1a seems to have broken into eastern and western subclades at about the expected time from the archaeology and linguistics. We can see the split between the Asian R1a1a and European R1a1a coinciding with the move east of the Urals to Sintashta.