diff --git a/OpenTelemetry.sln b/OpenTelemetry.sln
index 3b705899e..22e4bddf7 100644
--- a/OpenTelemetry.sln
+++ b/OpenTelemetry.sln
@@ -218,6 +218,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "customizing-the-sdk", "docs
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "OpenTelemetry.Tests.Stress", "test\OpenTelemetry.Tests.Stress\OpenTelemetry.Tests.Stress.csproj", "{2770158A-D220-414B-ABC6-179371323579}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "OpenTelemetry.Exporter.Prometheus.Tests", "test\OpenTelemetry.Exporter.Prometheus.Tests\OpenTelemetry.Exporter.Prometheus.Tests.csproj", "{380EE686-91F1-45B3-AEEB-755F0E5B068F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -448,6 +450,10 @@ Global
 		{2770158A-D220-414B-ABC6-179371323579}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{2770158A-D220-414B-ABC6-179371323579}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{2770158A-D220-414B-ABC6-179371323579}.Release|Any CPU.Build.0 = Release|Any CPU
+		{380EE686-91F1-45B3-AEEB-755F0E5B068F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{380EE686-91F1-45B3-AEEB-755F0E5B068F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{380EE686-91F1-45B3-AEEB-755F0E5B068F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{380EE686-91F1-45B3-AEEB-755F0E5B068F}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
diff --git a/src/OpenTelemetry.Exporter.Prometheus/AssemblyInfo.cs b/src/OpenTelemetry.Exporter.Prometheus/AssemblyInfo.cs
index 36f813dca..fd06ffcdf 100644
--- a/src/OpenTelemetry.Exporter.Prometheus/AssemblyInfo.cs
+++ b/src/OpenTelemetry.Exporter.Prometheus/AssemblyInfo.cs
@@ -17,6 +17,8 @@ using System.Runtime.CompilerServices;
 
 #if SIGNED
 [assembly: InternalsVisibleTo("Benchmarks, PublicKey=002400000480000094000000060200000024000052534131000400000100010051c1562a090fb0c9f391012a32198b5e5d9a60e9b80fa2d7b434c9e5ccb7259bd606e66f9660676afc6692b8cdc6793d190904551d2103b7b22fa636dcbb8208839785ba402ea08fc00c8f1500ccef28bbf599aa64ffb1e1d5dc1bf3420a3777badfe697856e9d52070a50c3ea5821c80bef17ca3acffa28f89dd413f096f898")]
+[assembly: InternalsVisibleTo("OpenTelemetry.Exporter.Prometheus.Tests, PublicKey=002400000480000094000000060200000024000052534131000400000100010051c1562a090fb0c9f391012a32198b5e5d9a60e9b80fa2d7b434c9e5ccb7259bd606e66f9660676afc6692b8cdc6793d190904551d2103b7b22fa636dcbb8208839785ba402ea08fc00c8f1500ccef28bbf599aa64ffb1e1d5dc1bf3420a3777badfe697856e9d52070a50c3ea5821c80bef17ca3acffa28f89dd413f096f898")]
 #else
 [assembly: InternalsVisibleTo("Benchmarks")]
+[assembly: InternalsVisibleTo("OpenTelemetry.Exporter.Prometheus.Tests")]
 #endif
diff --git a/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusExporterExtensions.cs b/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusExporterExtensions.cs
index 74d5c4345..1b1462755 100644
--- a/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusExporterExtensions.cs
+++ b/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusExporterExtensions.cs
@@ -14,6 +14,7 @@
 // limitations under the License.
 // </copyright>
 
+using System;
 using System.Globalization;
 using System.IO;
 using System.Threading.Tasks;
@@ -36,17 +37,20 @@ namespace OpenTelemetry.Exporter.Prometheus
         private const string PrometheusHistogramBucketLabelPositiveInfinity = "+Inf";
         private const string PrometheusHistogramBucketLabelLessThan = "le";
 
+        private static readonly Func<DateTimeOffset> DefaultGetUtcNowDateTimeOffset = () => DateTimeOffset.UtcNow;
+
         /// <summary>
         /// Serialize metrics to prometheus format.
         /// </summary>
         /// <param name="exporter"><see cref="PrometheusExporter"/>.</param>
         /// <param name="writer">StreamWriter to write to.</param>
+        /// <param name="getUtcNowDateTimeOffset">Optional function to resolve the current date &amp; time.</param>
         /// <returns><see cref="Task"/> to await the operation.</returns>
-        public static async Task WriteMetricsCollection(this PrometheusExporter exporter, StreamWriter writer)
+        public static async Task WriteMetricsCollection(this PrometheusExporter exporter, StreamWriter writer, Func<DateTimeOffset> getUtcNowDateTimeOffset = null)
         {
             foreach (var metric in exporter.Metrics)
             {
-                var builder = new PrometheusMetricBuilder()
+                var builder = new PrometheusMetricBuilder(getUtcNowDateTimeOffset ?? DefaultGetUtcNowDateTimeOffset)
                     .WithName(metric.Name)
                     .WithDescription(metric.Description);
 
diff --git a/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusMetricBuilder.cs b/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusMetricBuilder.cs
index 751a7be50..d4166a1f5 100644
--- a/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusMetricBuilder.cs
+++ b/src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusMetricBuilder.cs
@@ -59,11 +59,17 @@ namespace OpenTelemetry.Exporter.Prometheus
         };
 
         private readonly ICollection<PrometheusMetricValueBuilder> values = new List<PrometheusMetricValueBuilder>();
+        private readonly Func<DateTimeOffset> getUtcNowDateTimeOffset;
 
         private string name;
         private string description;
         private string type;
 
+        public PrometheusMetricBuilder(Func<DateTimeOffset> getUtcNowDateTimeOffset)
+        {
+            this.getUtcNowDateTimeOffset = getUtcNowDateTimeOffset;
+        }
+
         public PrometheusMetricBuilder WithName(string name)
         {
             this.name = name;
@@ -141,7 +147,7 @@ namespace OpenTelemetry.Exporter.Prometheus
             // ] value [ timestamp ]
             // In the sample syntax:
 
-            var now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds().ToString(CultureInfo.InvariantCulture);
+            var now = this.getUtcNowDateTimeOffset().ToUnixTimeMilliseconds().ToString(CultureInfo.InvariantCulture);
 
             foreach (var m in this.values)
             {
diff --git a/test/OpenTelemetry.Exporter.Prometheus.Tests/OpenTelemetry.Exporter.Prometheus.Tests.csproj b/test/OpenTelemetry.Exporter.Prometheus.Tests/OpenTelemetry.Exporter.Prometheus.Tests.csproj
new file mode 100644
index 000000000..bed1d487a
--- /dev/null
+++ b/test/OpenTelemetry.Exporter.Prometheus.Tests/OpenTelemetry.Exporter.Prometheus.Tests.csproj
@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Description>Unit test project for Prometheus Exporter for OpenTelemetry</Description>
+    <TargetFrameworks>netcoreapp3.1;net5.0</TargetFrameworks>
+    <TargetFrameworks Condition="$(OS) == 'Windows_NT'">$(TargetFrameworks);net461</TargetFrameworks>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="$(MicrosoftNETTestSdkPkgVer)" />
+    <PackageReference Include="Moq" Version="$(MoqPkgVer)" />
+    <PackageReference Include="xunit" Version="$(XUnitPkgVer)" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="$(XUnitRunnerVisualStudioPkgVer)">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
+    <DotNetCliToolReference Include="dotnet-xunit" Version="$(DotNetXUnitCliVer)" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Prometheus\OpenTelemetry.Exporter.Prometheus.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="$(RepoRoot)\test\OpenTelemetry.Tests\Shared\TestExporter.cs" Link="Includes\TestExporter.cs" />
+  </ItemGroup>
+
+</Project>
diff --git a/test/OpenTelemetry.Exporter.Prometheus.Tests/PrometheusExporterExtensionsTests.cs b/test/OpenTelemetry.Exporter.Prometheus.Tests/PrometheusExporterExtensionsTests.cs
new file mode 100644
index 000000000..3762ddb13
--- /dev/null
+++ b/test/OpenTelemetry.Exporter.Prometheus.Tests/PrometheusExporterExtensionsTests.cs
@@ -0,0 +1,77 @@
+// <copyright file="PrometheusExporterExtensionsTests.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Metrics;
+using System.IO;
+using System.Text;
+using OpenTelemetry.Metrics;
+using OpenTelemetry.Tests;
+using Xunit;
+
+namespace OpenTelemetry.Exporter.Prometheus.Tests
+{
+    public sealed class PrometheusExporterExtensionsTests
+    {
+        [Fact]
+        public void WriteMetricsCollectionTest()
+        {
+            var tags = new KeyValuePair<string, object>[]
+            {
+                new KeyValuePair<string, object>("key1", "value1"),
+                new KeyValuePair<string, object>("key2", "value2"),
+            };
+
+            using var provider = Sdk.CreateMeterProviderBuilder()
+                .AddSource("TestMeter")
+                .AddReader(new BaseExportingMetricReader(new TestExporter<Metric>(RunTest)))
+                .Build();
+
+            using var meter = new Meter("TestMeter", "0.0.1");
+
+            var counter = meter.CreateCounter<int>("counter");
+
+            counter.Add(100, tags);
+
+            var testCompleted = false;
+
+            // Invokes the TestExporter which will invoke RunTest
+            provider.ForceFlush(3000);
+
+            Assert.True(testCompleted);
+
+            void RunTest(Batch<Metric> metrics)
+            {
+                using PrometheusExporter prometheusExporter = new PrometheusExporter(new PrometheusExporterOptions());
+
+                prometheusExporter.Metrics = metrics;
+
+                using MemoryStream ms = new MemoryStream();
+                using (StreamWriter writer = new StreamWriter(ms))
+                {
+                    PrometheusExporterExtensions.WriteMetricsCollection(prometheusExporter, writer, () => new DateTimeOffset(2021, 9, 30, 22, 30, 0, TimeSpan.Zero)).GetAwaiter().GetResult();
+                }
+
+                Assert.Equal(
+                    "# TYPE counter counter\ncounter{key1=\"value1\",key2=\"value2\"} 100 1633041000000\n",
+                    Encoding.UTF8.GetString(ms.ToArray()));
+
+                testCompleted = true;
+            }
+        }
+    }
+}