Fix RemainingTime returning negative values

Libraries/src/Amazon.Lambda.RuntimeSupport/Context/LambdaContext.cs

@@ -21,7 +21,7 @@ namespace Amazon.Lambda.RuntimeSupport
 {
     internal class LambdaContext : ILambdaContext
     {
-        private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1);
+        private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
 
         private LambdaEnvironment _lambdaEnvironment;
         private RuntimeApiHeaders _runtimeApiHeaders;
@@ -69,6 +69,6 @@ public LambdaContext(RuntimeApiHeaders runtimeApiHeaders, LambdaEnvironment lamb
 
         public int MemoryLimitInMB => _memoryLimitInMB;
 
-        public TimeSpan RemainingTime => TimeSpan.FromMilliseconds(_deadlineMs - (DateTime.Now - UnixEpoch).TotalMilliseconds);
+        public TimeSpan RemainingTime => TimeSpan.FromMilliseconds(_deadlineMs - (DateTime.UtcNow - UnixEpoch).TotalMilliseconds);
     }
 }

Libraries/test/Amazon.Lambda.RuntimeSupport.Tests/Amazon.Lambda.RuntimeSupport.UnitTests/LambdaContextTests.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using Xunit;
+
+namespace Amazon.Lambda.RuntimeSupport.UnitTests
+{
+    public class LambdaContextTests
+    {
+        private readonly TestEnvironmentVariables _environmentVariables;
+
+        public LambdaContextTests()
+        {
+            _environmentVariables = new TestEnvironmentVariables();
+        }
+
+        [Fact]
+        public void RemainingTimeIsPositive()
+        {
+            var deadline = DateTimeOffset.UtcNow.AddHours(1);
+            var deadlineMs = deadline.ToUnixTimeMilliseconds().ToString(CultureInfo.InvariantCulture);
+
+            var headers = new Dictionary<string, IEnumerable<string>>
+            {
+                ["Lambda-Runtime-Aws-Request-Id"] = new[] { Guid.NewGuid().ToString() },
+                ["Lambda-Runtime-Deadline-Ms"] = new[] { deadlineMs },
+                ["Lambda-Runtime-Invoked-Function-Arn"] = new[] { "my-function-arn" }
+            };
+
+            var runtimeApiHeaders = new RuntimeApiHeaders(headers);
+            var lambdaEnvironment = new LambdaEnvironment(_environmentVariables);
+
+            var context = new LambdaContext(runtimeApiHeaders, lambdaEnvironment);
+
+            Assert.True(context.RemainingTime >= TimeSpan.Zero, $"Remaining time is not a positive value: {context.RemainingTime}");
+        }
+    }
+}