private void ResolveMaxAge(string headers, DateTimeOffset layoutRetrievedTime)
{
int startIndex = headers.IndexOf(KeyMaxAge, 0, headers.Length, StringComparison.Ordinal) + KeyMaxAge.Length + 1;
int endIndex = headers.IndexOf(';', startIndex, headers.Length - startIndex);
string maxAgeAsString = headers.Substring(startIndex, endIndex - startIndex);
double maxAgeAsDouble = double.Parse(maxAgeAsString);
ValidTill = layoutRetrievedTime + TimeSpan.FromSeconds(maxAgeAsDouble);
}