Hamilton's assessment of the death of Samuel Cowan was that it probably occurred in 1776. We know that Ann Walker Cowan was taken captive and her brother Samuel killed in 1777, so 1776 seems likely for Samuel Cowan's death. More pertinently, court records confirm that Samuel father of John died in 1776 (see below).
Eldest son John is thought to have been born c1765; he appears to have been a a minor at the time of his fathers death in 1776, and inherited his fathers property about 1782. That's consistent with a c1765 DOB. This implies that Ann and Samuel married prior to 1765. Ann's parents married in 1734 White, 1902; if she were the eldest child (there is no specific reason to believe that she was the eldest child), she would have been born no earlier than 1735, and would have reached marriagable age c1755. Thus, the DOM for this couple probably lies between 1755 and 1765.
If she married as early as 1755, the POM could be Borden's Grant in modern rockbridge Co, VA, as that's where her parents were living at the time. Her parents moved to NC c1756, and that's the more likely POM.