public static async Task Run()
{
    var ca2012Task = GetDateHeader("http://testca2012.cryptopro.ru");
    var caTask = GetDateHeader("http://testca.cryptopro.ru");
    var now1 = DateTimeOffset.Now;
    var dates = await Task.WhenAll(ca2012Task, caTask);
    var now2 = DateTimeOffset.Now;
    Console.WriteLine(now1);
    Console.WriteLine(dates[0]);
    Console.WriteLine(dates[1]);
    Console.WriteLine(now2);
}

private static async Task<DateTimeOffset?> GetDateHeader(string url)
{
    using var httpClient = new HttpClient();
    httpClient.BaseAddress = new Uri(url);
    var response = await httpClient.GetAsync((Uri?)null);
    return response.Headers.Date;
}