Sorry Shetop for not responding sooner. Based on Jordanes the Goths had been living in Moesia/Thrace/Dacia before they Settled above the Sea of Pontus (Modern West Ukraine). But we see the major migration at the beginning of the Gothic war south of the Danube. If I have to pick an exact date I will say 376 AD as it is the start of the Gothic War, Pannonia was settled sometime after that and the Goths were there for ~100 years before the establishment of the Ostrogothic Kingdom and their move into Dalmatia.
Jordanes lists many settlements in the Balkans by the Goths
(38) We read that on their first migration the Goths dwelt in the land of Scythia near Lake Maeotis. On the second migration they went to Moesia, Thrace and Dacia, and after their third they dwelt again in Scythia, above the Sea of Pontus. Nor do we find anywhere in their written records legends which tell of their subjection to slavery in Britain or in some other island, or of their redemption by a certain man at the cost of a single horse. Of course if anyone in our city says that the Goths had an origin different from that I have related, let him object. For myself, I prefer to believe what I have read, rather than put trust in old wives' tales.
And a little later
Now when the Goths saw the Gepidae defending for themselves the territory of the Huns and the people of the Huns dwelling again in their ancient abodes, they preferred to ask for lands from the Roman Empire, rather than invade the lands of others with danger to themselves. So they received Pannonia, which stretches in a long plain, being bounded on the east by Upper Moesia, on the south by Dalmatia, on the west by Noricum and on the north by the Danube.
Now look at that list of settled areas and look at exactly where we see an increase in the amont of I2a1 and a decrease in the areas where they are not listed as having Settled. Don't let that spot in Eastern Germany/West Czech Republic throw you off or any of it found West of Italy, that is all I2a1a (not Din) and we are talking about I2a1b(L621 to be precise)
And another Wielbark/Chernyakhov map
and a little more on the 'Chernyakhov' from wikipedia
Migration
Whilst acknowledging the mixed origins of the Chernyakiv culture, Peter Heather suggests that the culture is ultimately a reflection of the Goths' domination of the Pontic area. He cites literary sources that attest that the Goths were the centre of political attention at this time.[16] In particular, the culture's development corresponds well with Jordanes' tale of Gothic migration from Gothiscandza to Oium, under the leadership of Filimer. Moreoever, he highlights that crucial external influences that catalysed Chernyakhov cultural development derived from the Wielbark culture. Originating in the mid-1st century, it spread from south of the Baltic Sea (from territory around later Pomerania) down the Vistula in the 2nd and 3rd centuries. Wielbark elements are prominent in the Chernyakhov zone, such as typical 'Germanic' pottery, brooch types and female costume, and, in particular, weaponless bi-ritual burials. Although cultures may spread without substantial population movements, Heather draws attention to a decrease in the number of settlements in the original Pomeranian Wielbark heartland as evidence of a significant population movement. Combined with Jordanes' account, Heather concludes that a movement of Goths (and other east Germanic groups such as Heruli and Gepids) "played a major role in the creation of the Cernjachove culture".[17] He clarifies that this movement was not a single, royal-led, migration, but was rather accomplished by a series of small, sometimes mutually antagonistic groups.[18]
The part I bolded there is a huge point for the background of these people being NON R1a. R1a=Indo Eurpeans=Kurgans=Buried WITH weapons. Weaponless burials is a huge boon for I2a-Din being the primary Y DNA Haplogroup of the Goths.