Well-known type operations for Timestamp and Duration (but not JSON formatting).

While I've provided operators, I haven't yet provided the method equivalents. It's not clear to me that
they're actually a good idea, while we're really targeting C# developers who definitely *can* use the user-defined operators.

csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj

@@ -97,6 +97,8 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="TestCornerCases.cs" />
     <Compile Include="TestProtos\UnittestWellKnownTypes.cs" />
+    <Compile Include="WellKnownTypes\DurationTest.cs" />
+    <Compile Include="WellKnownTypes\TimestampTest.cs" />
     <Compile Include="WellKnownTypes\WrappersTest.cs" />
   </ItemGroup>
   <ItemGroup>

csharp/src/Google.Protobuf.Test/WellKnownTypes/DurationTest.cs

+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using NUnit.Framework;
+using System;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    public class DurationTest
+    {
+        [Test]
+        public void ToTimeSpan()
+        {
+            Assert.AreEqual(TimeSpan.FromSeconds(1), new Duration { Seconds = 1 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromSeconds(-1), new Duration { Seconds = -1 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromMilliseconds(1), new Duration { Nanos = 1000000 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromMilliseconds(-1), new Duration { Nanos = -1000000 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromTicks(1), new Duration { Nanos = 100 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromTicks(-1), new Duration { Nanos = -100 }.ToTimeSpan());
+
+            // Rounding is towards 0
+            Assert.AreEqual(TimeSpan.FromTicks(2), new Duration { Nanos = 250 }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromTicks(-2), new Duration { Nanos = -250 }.ToTimeSpan());
+
+            // Non-normalized durations
+            Assert.AreEqual(TimeSpan.FromSeconds(3), new Duration { Seconds = 1, Nanos = 2 * Duration.NanosecondsPerSecond }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromSeconds(1), new Duration { Seconds = 3, Nanos = -2 * Duration.NanosecondsPerSecond }.ToTimeSpan());
+            Assert.AreEqual(TimeSpan.FromSeconds(-1), new Duration { Seconds = 1, Nanos = -2 * Duration.NanosecondsPerSecond }.ToTimeSpan());
+        }
+
+        [Test]
+        public void Addition()
+        {
+            Assert.AreEqual(new Duration { Seconds = 2, Nanos = 100000000 },
+                new Duration { Seconds = 1, Nanos = 600000000 } + new Duration { Nanos = 500000000 });
+            Assert.AreEqual(new Duration { Seconds = -2, Nanos = -100000000 },
+                new Duration { Seconds = -1, Nanos = -600000000 } + new Duration { Nanos = -500000000 });
+            Assert.AreEqual(new Duration { Seconds = 1, Nanos = 100000000 },
+                new Duration { Seconds = 1, Nanos = 600000000 } + new Duration { Nanos = -500000000 });
+
+            // Non-normalized durations, or non-normalized intermediate results
+            Assert.AreEqual(new Duration { Seconds = 1 },
+                new Duration { Seconds = 1, Nanos = -500000000 } + new Duration { Nanos = 500000000 });
+
+            Assert.AreEqual(new Duration { Nanos = -900000000 },
+                new Duration { Seconds = -1, Nanos = -100000000 } + new Duration { Nanos = 200000000 });
+            Assert.AreEqual(new Duration { Nanos = 900000000 },
+                new Duration { Seconds = 1, Nanos = 100000000 } + new Duration { Nanos = -200000000 });
+        }
+
+        [Test]
+        public void Subtraction()
+        {
+            Assert.AreEqual(new Duration { Seconds = 1, Nanos = 100000000 },
+                new Duration { Seconds = 1, Nanos = 600000000 } - new Duration { Nanos = 500000000 });
+            Assert.AreEqual(new Duration { Seconds = -1, Nanos = -100000000 },
+                new Duration { Seconds = -1, Nanos = -600000000 } - new Duration { Nanos = -500000000 });
+            Assert.AreEqual(new Duration { Seconds = 2, Nanos = 100000000 },
+                new Duration { Seconds = 1, Nanos = 600000000 } - new Duration { Nanos = -500000000 });
+
+            // Non-normalized durations
+            Assert.AreEqual(new Duration(),
+                new Duration { Seconds = 1, Nanos = -500000000 } - new Duration { Nanos = 500000000 });
+            Assert.AreEqual(new Duration { Seconds = 1 },
+                new Duration { Nanos = 2000000000 } - new Duration { Nanos = 1000000000 });
+        }
+
+        [Test]
+        public void FromTimeSpan()
+        {
+            Assert.AreEqual(new Duration { Seconds = 1 }, Duration.FromTimeSpan(TimeSpan.FromSeconds(1)));
+            Assert.AreEqual(new Duration { Nanos = Duration.NanosecondsPerTick }, Duration.FromTimeSpan(TimeSpan.FromTicks(1)));
+        }
+    }
+}

csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs

+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using NUnit.Framework;
+using System;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    public class TimestampTest
+    {
+        [Test]
+        public void FromAndToDateTime()
+        {
+            DateTime utcMin = DateTime.SpecifyKind(DateTime.MinValue, DateTimeKind.Utc);
+            DateTime utcMax = DateTime.SpecifyKind(DateTime.MaxValue, DateTimeKind.Utc);
+            AssertRoundtrip(new Timestamp { Seconds = -62135596800 }, utcMin);
+            AssertRoundtrip(new Timestamp { Seconds = 253402300799, Nanos = 999999900 }, utcMax);
+            AssertRoundtrip(new Timestamp(), new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc));
+            AssertRoundtrip(new Timestamp { Nanos = 1000000}, new DateTime(1970, 1, 1, 0, 0, 0, 1, DateTimeKind.Utc));
+            AssertRoundtrip(new Timestamp { Seconds = -1, Nanos = 999000000 }, new DateTime(1969, 12, 31, 23, 59, 59, 999, DateTimeKind.Utc));
+            AssertRoundtrip(new Timestamp { Seconds = 3600 }, new DateTime(1970, 1, 1, 1, 0, 0, DateTimeKind.Utc));
+            AssertRoundtrip(new Timestamp { Seconds = -3600 }, new DateTime(1969, 12, 31, 23, 0, 0, DateTimeKind.Utc));
+        }
+
+        [Test]
+        public void ToDateTimeTruncation()
+        {
+            var t1 = new Timestamp { Seconds = 1, Nanos = 1000000 + Duration.NanosecondsPerTick - 1 };
+            Assert.AreEqual(new DateTime(1970, 1, 1, 0, 0, 1, DateTimeKind.Utc).AddMilliseconds(1), t1.ToDateTime());
+
+            var t2 = new Timestamp { Seconds = -1, Nanos = 1000000 + Duration.NanosecondsPerTick - 1 };
+            Assert.AreEqual(new DateTime(1969, 12, 31, 23, 59, 59).AddMilliseconds(1), t2.ToDateTime());
+        }
+
+        private static void AssertRoundtrip(Timestamp timestamp, DateTime dateTime)
+        {
+            Assert.AreEqual(timestamp, Timestamp.FromDateTime(dateTime));
+            Assert.AreEqual(dateTime, timestamp.ToDateTime());
+            Assert.AreEqual(DateTimeKind.Utc, timestamp.ToDateTime().Kind);
+        }
+
+        [Test]
+        public void Arithmetic()
+        {
+            Timestamp t1 = new Timestamp { Seconds = 10000, Nanos = 5000 };
+            Timestamp t2 = new Timestamp { Seconds = 8000, Nanos = 10000 };
+            Duration difference = new Duration { Seconds = 1999, Nanos = Duration.NanosecondsPerSecond - 5000 };
+            Assert.AreEqual(difference, t1 - t2);
+            Assert.AreEqual(-difference, t2 - t1);
+            
+            Assert.AreEqual(t1, t2 + difference);
+            Assert.AreEqual(t2, t1 - difference);
+        }
+    }
+}

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -100,11 +100,14 @@
     <Compile Include="WellKnownTypes\Any.cs" />
     <Compile Include="WellKnownTypes\Api.cs" />
     <Compile Include="WellKnownTypes\Duration.cs" />
+    <Compile Include="WellKnownTypes\DurationPartial.cs" />
     <Compile Include="WellKnownTypes\Empty.cs" />
     <Compile Include="WellKnownTypes\FieldMask.cs" />
     <Compile Include="WellKnownTypes\SourceContext.cs" />
     <Compile Include="WellKnownTypes\Struct.cs" />
+    <Compile Include="WellKnownTypes\TimeExtensions.cs" />
     <Compile Include="WellKnownTypes\Timestamp.cs" />
+    <Compile Include="WellKnownTypes\TimestampPartial.cs" />
     <Compile Include="WellKnownTypes\Type.cs" />
     <Compile Include="WellKnownTypes\Wrappers.cs" />
     <Compile Include="WireFormat.cs" />

csharp/src/Google.Protobuf/WellKnownTypes/DurationPartial.cs

+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    // Manually-written partial class for the Duration well-known type,
+    // providing a conversion to TimeSpan and convenience operators.
+    public partial class Duration
+    {
+        public const int NanosecondsPerSecond = 1000000000;
+        public const int NanosecondsPerTick = 100;
+
+        /// <summary>
+        /// Converts this <see cref="Duration"/> to a <see cref="TimeSpan"/>.
+        /// </summary>
+        /// <remarks>If the duration is not a precise number of ticks, it is truncated towards 0.</remarks>
+        /// <returns>The value of this duration, as a <c>TimeSpan</c>.</returns>
+        public TimeSpan ToTimeSpan()
+        {
+            checked
+            {
+                long ticks = Seconds * TimeSpan.TicksPerSecond + Nanos / NanosecondsPerTick;
+                return TimeSpan.FromTicks(ticks);
+            }
+        }
+
+        /// <summary>
+        /// Converts the given <see cref="TimeSpan"/> to a <see cref="Duration"/>.
+        /// </summary>
+        /// <param name="timeSpan">The <c>TimeSpan</c> to convert.</param>
+        /// <returns>The value of the given <c>TimeSpan</c>, as a <c>Duration</c>.</returns>
+        public static Duration FromTimeSpan(TimeSpan timeSpan)
+        {
+            checked
+            {
+                long ticks = timeSpan.Ticks;
+                long seconds = ticks / TimeSpan.TicksPerSecond;
+                int nanos = (int) (ticks % TimeSpan.TicksPerSecond) * NanosecondsPerTick;
+                return new Duration { Seconds = seconds, Nanos = nanos };
+            }
+        }
+
+        /// <summary>
+        /// Returns the result of negating the duration. For example, the negation of 5 minutes is -5 minutes.
+        /// </summary>
+        /// <param name="value">The duration to negate. Must not be null.</param>
+        /// <returns>The negated value of this duration.</returns>
+        public static Duration operator -(Duration value)
+        {
+            Preconditions.CheckNotNull(value, "value");
+            checked
+            {
+                return Normalize(-value.Seconds, -value.Nanos);
+            }
+        }
+
+        /// <summary>
+        /// Adds the two specified <see cref="Duration"/> values together.
+        /// </summary>
+        /// <param name="lhs">The first value to add. Must not be null.</param>
+        /// <param name="rhs">The second value to add. Must not be null.</param>
+        /// <returns></returns>
+        public static Duration operator +(Duration lhs, Duration rhs)
+        {
+            Preconditions.CheckNotNull(lhs, "lhs");
+            Preconditions.CheckNotNull(rhs, "rhs");
+            checked
+            {
+                return Normalize(lhs.Seconds + rhs.Seconds, lhs.Nanos + rhs.Nanos);
+            }
+        }
+
+        /// <summary>
+        /// Subtracts one <see cref="Duration"/> from another.
+        /// </summary>
+        /// <param name="lhs">The duration to subtract from. Must not be null.</param>
+        /// <param name="rhs">The duration to subtract. Must not be null.</param>
+        /// <returns>The difference between the two specified durations.</returns>
+        public static Duration operator -(Duration lhs, Duration rhs)
+        {
+            Preconditions.CheckNotNull(lhs, "lhs");
+            Preconditions.CheckNotNull(rhs, "rhs");
+            checked
+            {
+                return Normalize(lhs.Seconds - rhs.Seconds, lhs.Nanos - rhs.Nanos);
+            }
+        }
+        
+        /// <summary>
+        /// Creates a duration with the normalized values from the given number of seconds and
+        /// nanoseconds, conforming with the description in the proto file.
+        /// </summary>
+        internal static Duration Normalize(long seconds, int nanoseconds)
+        {
+            // Ensure that nanoseconds is in the range (-1,000,000,000, +1,000,000,000)
+            int extraSeconds = nanoseconds / NanosecondsPerSecond;
+            seconds += extraSeconds;
+            nanoseconds -= extraSeconds * NanosecondsPerSecond;
+
+            // Now make sure that Sign(seconds) == Sign(nanoseconds) if Sign(seconds) != 0.
+            if (seconds < 0 && nanoseconds > 0)
+            {
+                seconds += 1;
+                nanoseconds -= NanosecondsPerSecond;
+            }
+            else if (seconds > 0 && nanoseconds < 0)
+            {
+                seconds -= 1;
+                nanoseconds += NanosecondsPerSecond;
+            }
+            return new Duration { Seconds = seconds, Nanos = nanoseconds };
+        }
+    }
+}

csharp/src/Google.Protobuf/WellKnownTypes/TimeExtensions.cs

+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    /// <summary>
+    /// Extension methods on BCL time-related types, converting to protobuf types.
+    /// </summary>
+    public static class TimeExtensions
+    {
+        public static Timestamp ToTimestamp(this DateTime dateTime)
+        {
+            return Timestamp.FromDateTime(dateTime);
+        }
+
+        public static Timestamp ToTimestamp(this DateTimeOffset dateTimeOffset)
+        {
+            return Timestamp.FromDateTimeOffset(dateTimeOffset);
+        }
+
+        public static Duration ToDuration(this TimeSpan timeSpan)
+        {
+            return Duration.FromTimeSpan(timeSpan);
+        }
+    }
+}

csharp/src/Google.Protobuf/WellKnownTypes/TimestampPartial.cs

+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    public partial class Timestamp
+    {
+        private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        private static readonly long BclSecondsAtUnixEpoch = UnixEpoch.Ticks / TimeSpan.TicksPerSecond;
+
+        /// <summary>
+        /// Returns the difference between one <see cref="Timestamp"/> and another, as a <see cref="Duration"/>.
+        /// </summary>
+        /// <param name="lhs">The timestamp to subtract from. Must not be null.</param>
+        /// <param name="rhs">The timestamp to subtract. Must not be null.</param>
+        /// <returns>The difference between the two specified timestamps.</returns>
+        public static Duration operator -(Timestamp lhs, Timestamp rhs)
+        {
+            Preconditions.CheckNotNull(lhs, "lhs");
+            Preconditions.CheckNotNull(rhs, "rhs");
+            checked
+            {
+                return Duration.Normalize(lhs.Seconds - rhs.Seconds, lhs.Nanos - rhs.Nanos);
+            }
+        }
+
+        /// <summary>
+        /// Adds a <see cref="Duration"/> to a <see cref="Timestamp"/>, to obtain another <c>Timestamp</c>.
+        /// </summary>
+        /// <param name="lhs">The timestamp to add the duration to. Must not be null.</param>
+        /// <param name="rhs">The duration to add. Must not be null.</param>
+        /// <returns>The result of adding the duration to the timestamp.</returns>
+        public static Timestamp operator +(Timestamp lhs, Duration rhs)
+        {
+            Preconditions.CheckNotNull(lhs, "lhs");
+            Preconditions.CheckNotNull(rhs, "rhs");
+            checked
+            {
+                return Normalize(lhs.Seconds + rhs.Seconds, lhs.Nanos + rhs.Nanos);
+            }
+        }
+
+        /// <summary>
+        /// Subtracts a <see cref="Duration"/> from a <see cref="Timestamp"/>, to obtain another <c>Timestamp</c>.
+        /// </summary>
+        /// <param name="lhs">The timestamp to subtract the duration from. Must not be null.</param>
+        /// <param name="rhs">The duration to subtract.</param>
+        /// <returns>The result of subtracting the duration from the timestamp.</returns>
+        public static Timestamp operator -(Timestamp lhs, Duration rhs)
+        {
+            Preconditions.CheckNotNull(lhs, "lhs");
+            Preconditions.CheckNotNull(rhs, "rhs");
+            checked
+            {
+                return Normalize(lhs.Seconds - rhs.Seconds, lhs.Nanos - rhs.Nanos);
+            }
+        }
+
+        /// <summary>
+        /// Converts this timestamp into a <see cref="DateTime"/>.
+        /// </summary>
+        /// <remarks>
+        /// The resulting <c>DateTime</c> will always have a <c>Kind</c> of <c>Utc</c>.
+        /// If the timestamp is not a precise number of ticks, it will be truncated towards the start
+        /// of time. For example, a timestamp with a <see cref="Nanos"/> value of 99 will result in a
+        /// <see cref="DateTime"/> value precisely on a second.
+        /// </remarks>
+        /// <returns>This timestamp as a <c>DateTime</c>.</returns>
+        public DateTime ToDateTime()
+        {
+            return UnixEpoch.AddSeconds(Seconds).AddTicks(Nanos / Duration.NanosecondsPerTick);
+        }
+
+        /// <summary>
+        /// Converts this timestamp into a <see cref="DateTimeOffset"/>.
+        /// </summary>
+        /// <remarks>
+        /// The resulting <c>DateTimeOffset</c> will always have an <c>Offset</c> of zero.
+        /// If the timestamp is not a precise number of ticks, it will be truncated towards the start
+        /// of time. For example, a timestamp with a <see cref="Nanos"/> value of 99 will result in a
+        /// <see cref="DateTimeOffset"/> value precisely on a second.
+        /// </remarks>
+        /// <returns>This timestamp as a <c>DateTimeOffset</c>.</returns>
+        public DateTimeOffset ToDateTimeOffset()
+        {
+            return new DateTimeOffset(ToDateTime(), TimeSpan.Zero);
+        }
+
+        /// <summary>
+        /// Converts the specified <see cref="DateTime"/> to a <see cref="Timestamp"/>.
+        /// </summary>
+        /// <param name="dateTime"></param>
+        /// <exception cref="ArgumentException">The <c>Kind</c> of <paramref name="dateTime"/> is not <c>DateTimeKind.Utc</c>.</exception>
+        /// <returns>The converted timestamp.</returns>
+        public static Timestamp FromDateTime(DateTime dateTime)
+        {
+            if (dateTime.Kind != DateTimeKind.Utc)
+            {
+                throw new ArgumentException("Conversion from DateTime to Timestamp requires the DateTime kind to be Utc", "dateTime");
+            }
+            // Do the arithmetic using DateTime.Ticks, which is always non-negative, making things simpler.
+            long secondsSinceBclEpoch = dateTime.Ticks / TimeSpan.TicksPerSecond;
+            int nanoseconds = (int)  (dateTime.Ticks % TimeSpan.TicksPerSecond) * Duration.NanosecondsPerTick;
+            return new Timestamp { Seconds = secondsSinceBclEpoch - BclSecondsAtUnixEpoch, Nanos = nanoseconds };
+        }
+
+        /// <summary>
+        /// Converts the specified <see cref="DateTimeOffset"/> to a <see cref="Timestamp"/>.
+        /// </summary>
+        /// <param name="dateTime"></param>
+        /// <returns>The converted timestamp.</returns>
+        public static Timestamp FromDateTimeOffset(DateTimeOffset dateTimeOffset)
+        {
+            // We don't need to worry about this having negative ticks: DateTimeOffset is constrained to handle
+            // values whose *UTC* value is in the range of DateTime.
+            return FromDateTime(dateTimeOffset.UtcDateTime);
+        }
+
+        private static Timestamp Normalize(long seconds, int nanoseconds)
+        {
+            int extraSeconds = nanoseconds / Duration.NanosecondsPerSecond;
+            seconds += extraSeconds;
+            nanoseconds -= extraSeconds * Duration.NanosecondsPerSecond;
+
+            if (nanoseconds < 0)
+            {
+                nanoseconds += Duration.NanosecondsPerSecond;
+                seconds--;
+            }
+            return new Timestamp { Seconds = seconds, Nanos = nanoseconds };
+        }
+    }
+}