By most accounts, David lived his entire life in the sixth century, that is the 500s, but that is far from firm. If we were bound his life from the earliest possible dates to the latest it would stretch from 462 to 601. He was a native of Wales, not someone who came from somewhere else, like St Patrick, who first went to Ireland as a slave before being sent back as a herald of the Gospel. St David's mother, Non, is also revered as a saint, as is his teacher, Paulinus. St David's Cathedral stands on the site of a monastery he founded.
Most of what we know about St David is taken from the Buchedd Dewi, a hagiography written by Rhygyfarch in the late 11th century. While it is probably true that some of this account of David's life was taken from documents the author found in an archive, it is almost certainly not true for the entire account. Rather than finding this disturbing in any way, this is where things take a peculiarly Welsh twist: in Rhygyfarch's day the Welsh church was seeking to establish some independence for itself (something that, at least in my view, would've been healthy given the subsequent history of Christianity in Wales). The Welsh church had refused the Roman rite until the 8th century. In the 11th century the Welsh church sought a metropolitan status equal to that of Canterbury. This is likely the reason for the apocryphal story of David going on pilgrimage to Jerusalem where he was anointed as an archbishop by the patriarch.
David can truly be said to have served as bishop and, later, as archbishop. He established several monastic communities and authored a monastic rule that was quite severe. For example, his rule prescribed that monks had to pull the plough themselves without draught animals. Further, it stipulated that the monks could only drink water and only eat bread with salt and herbs. David himself was known for his austere and ascetic lifestyle. He ate no meat and did not drink beer. It is partially due to this that St David's symbol, which is also the symbol of Wales, is the leek.
In a recent edition of the Roman Martyrology (2004), David's memorial (he listed with the Latin name Dávus) is confirmed as 1 March, the traditional date of his death. While David was Welsh and lived his life in Wales, his leadership resulted in many monks being sent to spread the Gospel throughout Wales, Ireland, Cornwall and other places as well.
Here is a link to a BBC article from 2010 on the now complete restoration of St David's shrine: "Pilgrim's progress for St David's Shrine." In addition, here's a BBC introductory piece on St David.
Dewi Sant yn gweddïo i ni