The number you see is a Javascript timestamp (number of milliseconds elapsed since January 1, 1970 00:00:00 UTC). You can test it in any browser console:

Following this article, you can convert back to DateTime
object:
public static DateTime ConvertFromUnixTimestamp(double timestamp)
{
DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
return origin.AddSeconds(timestamp / 1000); // convert from milliseconds to seconds
}
From the comment, I realize there is this method DateTimeOffset.FromUnixTimeMilliseconds as well:
// Convert Timestampt to DateTimeOffset
var time = DateTimeOffset.FromUnixTimeMilliseconds(1606813200000);
// Convert back to ISO string
var isoString = time.ToString("o");