DateTime.Now.ToString(CultureInfo.GetCultureInfo("ru-ru"))

Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("ru-ru");

static string GetCurrentDateTime()
{
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Russian Standard Time");
    return TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tz)
            .ToString(CultureInfo.GetCultureInfo("ru-ru"));
}