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/// <summary>Much faster than using Exiftool. In case GDI+ cannot decode the date string we use Exiftool.</summary>
private DateTime? GetOriginalDate( string sFileName )
{
    using ( FileStream stream = new FileStream( sFileName, FileMode.Open, FileAccess.Read ) )
    {
        using ( Image img = Image.FromStream( stream, false, false ) )
        {
            int[] date_tags = new int[] { 36867, 36868, 306 }; // tag numbers with dates
            string[] s1 = ( from x in date_tags where img.PropertyIdList.Contains( x ) select Encoding.ASCII.GetString( img.GetPropertyItem( x ).Value ).Replace( "\0", "" )).ToArray(); // get date as string without training \0
            DateTime d;
            DateTime?[] dd = ( from x in s1 where x.Trim().Length > 0
                select DateTime.TryParseExact( x, new string[] { "yyyy:MM:dd HH:mm:ss", "yyyy-MM-dd HH:mm:ss", "MM/dd/yyyy HH:mm:ss", "yyyy-MM-dd'T'HH:mm:sszzz" }, CultureInfo.InvariantCulture, DateTimeStyles.AllowWhiteSpaces | DateTimeStyles.AssumeLocal, out d ) ? d as DateTime? : 
                null ).ToArray(); // we see if we can parse all the date attributes found
            if ( dd.Where( x => !x.HasValue ).Count() > 0 )
                return GetOldestExifDateExifTool( sFileName ); // if there is something in the date attribute we cannot parse we ask exiftool.
            else
                return ( from x in dd where x.Value > new DateTime( 1990, 01, 01 ) && x.Value < DateTime.UtcNow select x ).Min(); // make sure we use a valid date range
        }
    }
}

/// <summary>Extracts the oldest possible EXIF date. Can process 3 files per second, very slow, will need 8 hours for 90.000 files.</summary>
private static DateTime? GetOldestExifDateExifTool( string sFile )
{
    Process oP = new Process();
    oP.EnableRaisingEvents = false;
    oP.StartInfo.CreateNoWindow = true;
    oP.StartInfo.LoadUserProfile = false;
    oP.StartInfo.RedirectStandardError = false;
    oP.StartInfo.RedirectStandardOutput = true;
    oP.StartInfo.RedirectStandardInput = true;
    oP.StartInfo.StandardErrorEncoding = null;
    oP.StartInfo.StandardOutputEncoding = Encoding.UTF8;
    oP.StartInfo.UseShellExecute = false;
    oP.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
    oP.StartInfo.FileName = @"exiftool.exe";
    oP.StartInfo.Arguments = "-s -s -EXIF:ModifyDate -EXIF:DateTimeOriginal -EXIF:CreateDate -d \"%Y-%m-%d %H:%M:%S\" -";
    oP.Start();
 
    byte[] image = File.ReadAllBytes( sFile );
    oP.StandardInput.BaseStream.Write( image, 0, image.Length );
    oP.StandardInput.BaseStream.Flush();
    oP.StandardInput.BaseStream.Close();
    string sStdOut = oP.StandardOutput.ReadToEnd();
    oP.WaitForExit();
 
    string[] datetags = new string[] { "DateTimeOriginal", "CreateDate", "ModifyDate" };
    string[] res1 = sStdOut.Split( new string[] { "\r\n" }, StringSplitOptions.RemoveEmptyEntries ); // split lines
    string[][] res2 = ( from x in res1 select x.Split( new char[] { ':' }, 2 ) ).ToArray(); // split after colon to separate attributes and values
    string[] res3 = ( from x in res2 where x.Length == 2 && datetags.Contains( x[ 0 ], StringComparer.InvariantCultureIgnoreCase ) select x[ 1 ] ).ToArray(); // only chose lines of date attributes
    DateTime d;
    DateTime?[] dd = ( from x in res3 select DateTime.TryParseExact( x, "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.AllowWhiteSpaces | DateTimeStyles.AssumeLocal, out d ) ? d as DateTime? : null ).ToArray();
    DateTime? oDate = ( from x in dd where x.HasValue && x > new DateTime( 1990, 01, 01 ) && x < DateTime.UtcNow select x ).Min();
    return oDate;
}