Added new project for .Net Compact Framework 3.5.

Cloned original GuerrillaNtp project and made nessisary changes.

New project was built using VS2008, latest version of VS compatible is CF.

GuerrillaNtp_CF/GuerrillaNtp_CF.csproj

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+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="3.5">
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProductVersion>9.0.30729</ProductVersion>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{6A920E41-4597-4312-A5D0-43CC0DE70EBC}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>GuerrillaNtp</RootNamespace>
+    <AssemblyName>GuerrillaNtp_CF</AssemblyName>
+    <ProjectTypeGuids>{4D628B5B-2FBC-4AA6-8C16-197242AEB884};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <PlatformFamilyName>PocketPC</PlatformFamilyName>
+    <PlatformID>b2c48bd2-963d-4549-9169-1fa021dce484</PlatformID>
+    <OSVersion>5.2</OSVersion>
+    <DeployDirSuffix>GuerrillaNtp_CF</DeployDirSuffix>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+    <NativePlatformName>Windows Mobile 6 Professional SDK</NativePlatformName>
+    <FormFactorID>
+    </FormFactorID>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE;$(PlatformFamilyName)</DefineConstants>
+    <NoStdLib>true</NoStdLib>
+    <NoConfig>true</NoConfig>
+    <ErrorReport>prompt</ErrorReport>
+    <FileAlignment>512</FileAlignment>
+    <WarningLevel>4</WarningLevel>
+    <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE;$(PlatformFamilyName)</DefineConstants>
+    <NoStdLib>true</NoStdLib>
+    <NoConfig>true</NoConfig>
+    <ErrorReport>prompt</ErrorReport>
+    <FileAlignment>512</FileAlignment>
+    <WarningLevel>4</WarningLevel>
+    <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="mscorlib" />
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="NtpClient.cs" />
+    <Compile Include="NtpException.cs" />
+    <Compile Include="NtpLeapIndicator.cs" />
+    <Compile Include="NtpMode.cs" />
+    <Compile Include="NtpPacket.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildBinPath)\Microsoft.CompactFramework.CSharp.targets" />
+  <ProjectExtensions>
+    <VisualStudio>
+      <FlavorProperties GUID="{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}">
+        <HostingProcess disable="1" />
+      </FlavorProperties>
+    </VisualStudio>
+  </ProjectExtensions>
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>

GuerrillaNtp_CF/NtpClient.cs

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+// Part of GuerrillaNtp: https://guerrillantp.machinezoo.com
+using System;
+using System.Net;
+using System.Net.Sockets;
+
+namespace GuerrillaNtp
+{
+    /// <summary>
+    /// Represents UDP socket used to communicate with RFC4330-compliant SNTP/NTP server.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// See <a href="https://guerrillantp.machinezoo.com/">project homepage</a> for guidance on how to use GuerrillaNtp.
+    /// Most applications should just call <see cref="M:GuerrillaNtp.NtpClient.GetCorrectionOffset" />
+    /// after instantiating this class. Method <see cref="M:GuerrillaNtp.NtpClient.Query" />
+    /// can be used to obtain additional details stored in reply <see cref="T:GuerrillaNtp.NtpPacket" />.
+    /// </para>
+    /// <para>
+    /// This class holds unmanaged resources (the socket) and callers are responsible
+    /// for calling <see cref="M:GuerrillaNtp.NtpClient.Dispose" /> when they are done,
+    /// perhaps by instantiating this class in <c>using</c> block.
+    /// </para>
+    /// <para>
+    /// It is application responsibility to be a good netizen,
+    /// which most importantly means using reasonable polling intervals
+    /// and exponential backoff when querying public NTP server.
+    /// </para>
+    /// </remarks>
+    public class NtpClient
+    {
+        readonly Socket socket;
+
+        /// <summary>
+        /// Creates new <see cref="T:GuerrillaNtp.NtpClient" /> from server endpoint.
+        /// </summary>
+        /// <param name="endpoint">Endpoint of the remote SNTP server.</param>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.#ctor(System.Net.IPAddress,System.Int32)" />
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Dispose" />
+        public NtpClient(IPEndPoint endpoint)
+        {
+            socket = new Socket(endpoint.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
+            try
+            {
+                socket.Connect(endpoint);
+            }
+            catch
+            {
+                throw;
+            }
+        }
+
+        /// <summary>
+        /// Creates new <see cref="T:GuerrillaNtp.NtpClient" /> from server's IP address and optional port.
+        /// </summary>
+        /// <param name="address">IP address of remote SNTP server, standard NTP port is used.</param>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.#ctor(System.Net.IPEndPoint)" />
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Dispose" />
+        public NtpClient(IPAddress address) : this(new IPEndPoint(address, 123)) { }
+
+        /// <summary>
+        /// Creates new <see cref="T:GuerrillaNtp.NtpClient" /> from server's IP address and optional port.
+        /// </summary>
+        /// <param name="address">IP address of remote SNTP server</param>
+        /// <param name="port">Port of remote SNTP server. Default is 123 (standard NTP port).</param>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.#ctor(System.Net.IPEndPoint)" />
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Dispose" />
+        public NtpClient(IPAddress address, int port) : this(new IPEndPoint(address, port)) { }
+
+        /// <summary>
+        /// Queries the SNTP server and returns correction offset.
+        /// </summary>
+        /// <remarks>
+        /// Use this method if you just want correction offset from the server.
+        /// Call <see cref="M:GuerrillaNtp.NtpClient.Query" /> to obtain <see cref="T:GuerrillaNtp.NtpPacket" />
+        /// with additional information besides <see cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" />.
+        /// </remarks>
+        /// <returns>
+        /// Offset that should be added to local time to match server time.
+        /// </returns>
+        /// <exception cref="T:GuerrillaNtp.NtpException">Thrown when the server responds with invalid reply packet.</exception>
+        /// <exception cref="T:System.Net.Sockets.SocketException">
+        /// Thrown when no reply is received before <see cref="P:GuerrillaNtp.NtpClient.Timeout" /> is reached
+        /// or when there is an error communicating with the server.
+        /// </exception>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Query" />
+        /// <seealso cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" />
+        public TimeSpan GetCorrectionOffset() { return Query().CorrectionOffset; }
+
+        /// <summary>
+        /// Queries the SNTP server with configurable <see cref="T:GuerrillaNtp.NtpPacket" /> request.
+        /// </summary>
+        /// <param name="request">SNTP request packet to use when querying the network time server.</param>
+        /// <returns>SNTP reply packet returned by the server.</returns>
+        /// <remarks>
+        /// <see cref="M:GuerrillaNtp.NtpPacket.#ctor" /> constructor
+        /// creates valid request packet, which you can further customize.
+        /// If you don't need any customization of the request packet, call <see cref="M:GuerrillaNtp.NtpClient.Query" /> instead.
+        /// Returned <see cref="T:GuerrillaNtp.NtpPacket" /> contains correction offset in
+        /// <see cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" /> property.
+        /// </remarks>
+        /// <exception cref="T:GuerrillaNtp.NtpException">
+        /// Thrown when the request packet is invalid or when the server responds with invalid reply packet.
+        /// </exception>
+        /// <exception cref="T:System.Net.Sockets.SocketException">
+        /// Thrown when no reply is received before <see cref="P:GuerrillaNtp.NtpClient.Timeout" /> is reached
+        /// or when there is an error communicating with the server.
+        /// </exception>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.GetCorrectionOffset" />
+        /// <seealso cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" />
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Query" />
+        public NtpPacket Query(NtpPacket request)
+        {
+            request.ValidateRequest();
+            socket.Send(request.Bytes);
+            var response = new byte[160];
+            int received = socket.Receive(response);
+            var truncated = new byte[received];
+            Array.Copy(response, truncated, received);
+            NtpPacket reply = new NtpPacket(truncated) { DestinationTimestamp = DateTime.UtcNow };
+            reply.ValidateReply(request);
+            return reply;
+        }
+
+        /// <summary>
+        /// Queries the SNTP server with default options.
+        /// </summary>
+        /// <remarks>
+        /// Use this method to obtain additional details from the returned <see cref="T:GuerrillaNtp.NtpPacket" />
+        /// besides <see cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" />.
+        /// If you just need the correction offset, call <see cref="M:GuerrillaNtp.NtpClient.GetCorrectionOffset" /> instead.
+        /// You can customize request packed by calling <see cref="M:GuerrillaNtp.NtpClient.Query(GuerrillaNtp.NtpPacket)" />.
+        /// </remarks>
+        /// <returns>SNTP reply packet returned by the server.</returns>
+        /// <exception cref="T:GuerrillaNtp.NtpException">Thrown when the server responds with invalid reply packet.</exception>
+        /// <exception cref="T:System.Net.Sockets.SocketException">
+        /// Thrown when no reply is received before <see cref="P:GuerrillaNtp.NtpClient.Timeout" /> is reached
+        /// or when there is an error communicating with the server.
+        /// </exception>
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.GetCorrectionOffset" />
+        /// <seealso cref="P:GuerrillaNtp.NtpPacket.CorrectionOffset" />
+        /// <seealso cref="M:GuerrillaNtp.NtpClient.Query(GuerrillaNtp.NtpPacket)" />
+       public NtpPacket Query() { return Query(new NtpPacket()); }
+    }
+}

GuerrillaNtp_CF/NtpException.cs

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+// Part of GuerrillaNtp: https://guerrillantp.machinezoo.com
+using System;
+
+namespace GuerrillaNtp
+{
+    /// <summary>
+    /// Represents errors that occur in SNTP packets or during SNTP operation.
+    /// </summary>
+    public class NtpException : Exception
+    {
+        /// <summary>
+        /// Gets the SNTP packet that caused this exception, if any.
+        /// </summary>
+        /// <value>
+        /// SNTP packet that caused this exception, usually reply packet,
+        /// or <c>null</c> if the error is not specific to any packet.
+        /// </value>
+        public NtpPacket Packet { get; private set; }
+
+        internal NtpException(NtpPacket packet, String message)
+            : base(message)
+        {
+            Packet = packet;
+        }
+    }
+}

GuerrillaNtp_CF/NtpLeapIndicator.cs

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+// Part of GuerrillaNtp: https://guerrillantp.machinezoo.com
+namespace GuerrillaNtp
+{
+    /// <summary>
+    /// Represents leap second warning from the server that instructs the client to add or remove leap second.
+    /// </summary>
+    /// <seealso cref="P:GuerrillaNtp.NtpPacket.LeapIndicator" />
+    public enum NtpLeapIndicator
+    {
+        /// <summary>
+        /// No leap second warning. No action required.
+        /// </summary>
+        NoWarning, 
+
+        /// <summary>
+        /// Warns the client that the last minute of the current day has 61 seconds.
+        /// </summary>
+        LastMinuteHas61Seconds, 
+
+        /// <summary>
+        /// Warns the client that the last minute of the current day has 59 seconds.
+        /// </summary>
+        LastMinuteHas59Seconds, 
+
+        /// <summary>
+        /// Special value indicating that the server clock is unsynchronized and the returned time is unreliable.
+        /// </summary>
+        AlarmCondition
+    }
+}

GuerrillaNtp_CF/NtpMode.cs

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+// Part of GuerrillaNtp: https://guerrillantp.machinezoo.com
+namespace GuerrillaNtp
+{
+    /// <summary>
+    /// Describes SNTP packet mode, i.e. client or server.
+    /// </summary>
+    /// <seealso cref="P:GuerrillaNtp.NtpPacket.Mode" />
+    public enum NtpMode
+    {
+        /// <summary>
+        /// Identifies client-to-server SNTP packet.
+        /// </summary>
+        Client = 3, 
+
+        /// <summary>
+        /// Identifies server-to-client SNTP packet.
+        /// </summary>
+        Server = 4, 
+    }
+}