Extension method to get the Sunday date from current date based on weekOffset
I think your code could be more succinctly and usefully (ie don't return string) written as:
    private static DateTimeOffset[] GetWeekRange(this DateTimeOffset d, int weekOffset = 0)
    {
        var weekStartDate = new DateTimeOffset(d.Date, d.Offset).AddDays(weekOffset * 7 - (int)d.DayOfWeek);
        var weekEndDate = weekStartDate.AddDays(7);
        return new [] { weekStartDate, weekEndDate };
    }

But then given that end is so easily calc'd from start, maybe just return start, forget the array