diff --git a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/exemplars/ExemplarSampler.java b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/exemplars/ExemplarSampler.java
index 1219b2a09..868bd11b2 100644
--- a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/exemplars/ExemplarSampler.java
+++ b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/exemplars/ExemplarSampler.java
@@ -40,13 +40,14 @@ public class ExemplarSampler {
   // to be overwritten by automatic exemplar sampling. exemplars.length == customExemplars.length
   private final AtomicBoolean acceptingNewExemplars = new AtomicBoolean(true);
   private final AtomicBoolean acceptingNewCustomExemplars = new AtomicBoolean(true);
+  private final LongSupplier currentTimeMillis;
 
   @Nullable
   private final SpanContext
       spanContext; // may be null, in that case SpanContextSupplier.getSpanContext() is used.
 
   public ExemplarSampler(ExemplarSamplerConfig config) {
-    this(config, null);
+    this(config, null, System::currentTimeMillis);
   }
 
   /**
@@ -58,15 +59,32 @@ public ExemplarSampler(ExemplarSamplerConfig config) {
    * SpanContextSupplier.getSpanContext()} is called to find a span context.
    */
   public ExemplarSampler(ExemplarSamplerConfig config, @Nullable SpanContext spanContext) {
+    this(config, spanContext, System::currentTimeMillis);
+  }
+
+  /**
+   * Constructor with an additional {code currentTimeMillis} argument for testing. This allows
+   * injecting a custom time source to make tests deterministic and avoid flaky tests caused by
+   * timing issues.
+   *
+   * @param config the exemplar sampler configuration
+   * @param spanContext the span context, may be null
+   * @param currentTimeMillis time source function that returns current time in milliseconds
+   */
+  public ExemplarSampler(
+      ExemplarSamplerConfig config,
+      @Nullable SpanContext spanContext,
+      LongSupplier currentTimeMillis) {
     this.config = config;
     this.exemplars = new Exemplar[config.getNumberOfExemplars()];
     this.customExemplars = new Exemplar[exemplars.length];
     this.spanContext = spanContext;
+    this.currentTimeMillis = currentTimeMillis;
   }
 
   public Exemplars collect() {
     // this may run in parallel with observe()
-    long now = System.currentTimeMillis();
+    long now = currentTimeMillis.getAsLong();
     List<Exemplar> result = new ArrayList<>(exemplars.length);
     for (int i = 0; i < customExemplars.length; i++) {
       Exemplar exemplar = customExemplars[i];
@@ -129,7 +147,7 @@ private long doObserve(double value) {
   }
 
   private long doObserveSingleExemplar(double value) {
-    long now = System.currentTimeMillis();
+    long now = currentTimeMillis.getAsLong();
     Exemplar current = exemplars[0];
     if (current == null
         || now - current.getTimestampMillis() > config.getMinRetentionPeriodMillis()) {
@@ -139,7 +157,7 @@ private long doObserveSingleExemplar(double value) {
   }
 
   private long doObserveSingleExemplar(double amount, Labels labels) {
-    long now = System.currentTimeMillis();
+    long now = currentTimeMillis.getAsLong();
     Exemplar current = customExemplars[0];
     if (current == null
         || now - current.getTimestampMillis() > config.getMinRetentionPeriodMillis()) {
@@ -149,7 +167,7 @@ private long doObserveSingleExemplar(double amount, Labels labels) {
   }
 
   private long doObserveWithUpperBounds(double value, double[] classicUpperBounds) {
-    long now = System.currentTimeMillis();
+    long now = currentTimeMillis.getAsLong();
     for (int i = 0; i < classicUpperBounds.length; i++) {
       if (value <= classicUpperBounds[i]) {
         Exemplar previous = exemplars[i];
@@ -165,7 +183,7 @@ private long doObserveWithUpperBounds(double value, double[] classicUpperBounds)
   }
 
   private long doObserveWithoutUpperBounds(double value) {
-    final long now = System.currentTimeMillis();
+    final long now = currentTimeMillis.getAsLong();
     Exemplar smallest = null;
     int smallestIndex = -1;
     Exemplar largest = null;
@@ -234,7 +252,7 @@ private long doObserveWithExemplar(double amount, Labels labels) {
 
   private long doObserveWithExemplarWithUpperBounds(
       double value, Labels labels, double[] classicUpperBounds) {
-    long now = System.currentTimeMillis();
+    long now = currentTimeMillis.getAsLong();
     for (int i = 0; i < classicUpperBounds.length; i++) {
       if (value <= classicUpperBounds[i]) {
         Exemplar previous = customExemplars[i];
@@ -250,7 +268,7 @@ private long doObserveWithExemplarWithUpperBounds(
   }
 
   private long doObserveWithExemplarWithoutUpperBounds(double amount, Labels labels) {
-    final long now = System.currentTimeMillis();
+    final long now = currentTimeMillis.getAsLong();
     int nullPos = -1;
     int oldestPos = -1;
     Exemplar oldest = null;
diff --git a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
index 54d2dcc67..04e4856b6 100644
--- a/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
+++ b/prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/Histogram.java
@@ -24,6 +24,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.DoubleAdder;
 import java.util.concurrent.atomic.LongAdder;
+import java.util.function.LongSupplier;
 import javax.annotation.Nullable;
 
 /**
@@ -71,6 +72,9 @@ public class Histogram extends StatefulMetric<DistributionDataPoint, Histogram.D
 
   @Nullable private final ExemplarSamplerConfig exemplarSamplerConfig;
 
+  // For testing: allows injecting a custom time source for exemplar sampling
+  @Nullable LongSupplier exemplarSamplerTimeSource = null;
+
   // Upper bounds for the classic histogram buckets. Contains at least +Inf.
   // An empty array indicates that this is a native histogram only.
   private final double[] classicUpperBounds;
@@ -199,7 +203,12 @@ public class DataPoint implements DistributionDataPoint {
 
     private DataPoint() {
       if (exemplarSamplerConfig != null) {
-        exemplarSampler = new ExemplarSampler(exemplarSamplerConfig);
+        if (exemplarSamplerTimeSource != null) {
+          exemplarSampler =
+              new ExemplarSampler(exemplarSamplerConfig, null, exemplarSamplerTimeSource);
+        } else {
+          exemplarSampler = new ExemplarSampler(exemplarSamplerConfig);
+        }
       } else {
         exemplarSampler = null;
       }
diff --git a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/exemplars/ExemplarSamplerConfigTestUtil.java b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/exemplars/ExemplarSamplerConfigTestUtil.java
index e22e9d6c6..44a2d8076 100644
--- a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/exemplars/ExemplarSamplerConfigTestUtil.java
+++ b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/exemplars/ExemplarSamplerConfigTestUtil.java
@@ -1,6 +1,7 @@
 package io.prometheus.metrics.core.exemplars;
 
 import java.lang.reflect.Field;
+import java.util.function.LongSupplier;
 
 public class ExemplarSamplerConfigTestUtil {
 
@@ -29,4 +30,11 @@ public static void setSampleIntervalMillis(Object metric, long value)
     ExemplarSamplerConfig config = getConfig(metric, "exemplarSamplerConfig");
     setRetentionPeriod(config, "sampleIntervalMillis", value);
   }
+
+  public static void setExemplarSamplerTimeSource(Object metric, LongSupplier timeSource)
+      throws NoSuchFieldException, IllegalAccessException {
+    Field timeSourceField = metric.getClass().getDeclaredField("exemplarSamplerTimeSource");
+    timeSourceField.setAccessible(true);
+    timeSourceField.set(metric, timeSource);
+  }
 }
diff --git a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
index 19e4cdc79..e3040c0b2 100644
--- a/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
+++ b/prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/HistogramTest.java
@@ -37,6 +37,7 @@
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
+import java.util.function.LongSupplier;
 import java.util.stream.Collectors;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -956,6 +957,21 @@ public void testDefaults() throws IOException {
 
   @Test
   public void testExemplarsClassicHistogram() throws Exception {
+    class MockTimeSource implements LongSupplier {
+      private long currentTimeMillis = 0;
+
+      @Override
+      public long getAsLong() {
+        return currentTimeMillis;
+      }
+
+      void advance(long millis) {
+        currentTimeMillis += millis;
+      }
+    }
+
+    MockTimeSource mockTime = new MockTimeSource();
+
     SpanContext spanContext =
         new SpanContext() {
           int callCount = 0;
@@ -990,6 +1006,7 @@ public void markCurrentSpanAsExemplar() {}
 
     long sampleIntervalMillis = 10;
     ExemplarSamplerConfigTestUtil.setSampleIntervalMillis(histogram, sampleIntervalMillis);
+    ExemplarSamplerConfigTestUtil.setExemplarSamplerTimeSource(histogram, mockTime);
     SpanContextSupplier.setSpanContext(spanContext);
 
     Exemplar ex1a = Exemplar.builder().value(0.5).spanId("spanId-1").traceId("traceId-1").build();
@@ -1020,7 +1037,9 @@ public void markCurrentSpanAsExemplar() {}
     assertThat(getExemplar(snapshot, Double.POSITIVE_INFINITY, "path", "/hello")).isNull();
     assertThat(getExemplar(snapshot, Double.POSITIVE_INFINITY, "path", "/world")).isNull();
 
-    Thread.sleep(sampleIntervalMillis + 1);
+    mockTime.advance(sampleIntervalMillis + 1);
+    // Small wait to let the scheduler re-enable accepting new exemplars
+    Thread.sleep(50);
     histogram.labelValues("/hello").observe(4.5);
     histogram.labelValues("/world").observe(4.5);
 
@@ -1036,13 +1055,16 @@ public void markCurrentSpanAsExemplar() {}
     assertExemplarEquals(ex2a, getExemplar(snapshot, Double.POSITIVE_INFINITY, "path", "/hello"));
     assertExemplarEquals(ex2b, getExemplar(snapshot, Double.POSITIVE_INFINITY, "path", "/world"));
 
-    Thread.sleep(sampleIntervalMillis + 1);
+    mockTime.advance(sampleIntervalMillis + 1);
+    Thread.sleep(50);
     histogram.labelValues("/hello").observe(1.5);
     histogram.labelValues("/world").observe(1.5);
-    Thread.sleep(sampleIntervalMillis + 1);
+    mockTime.advance(sampleIntervalMillis + 1);
+    Thread.sleep(50);
     histogram.labelValues("/hello").observe(2.5);
     histogram.labelValues("/world").observe(2.5);
-    Thread.sleep(sampleIntervalMillis + 1);
+    mockTime.advance(sampleIntervalMillis + 1);
+    Thread.sleep(50);
     histogram.labelValues("/hello").observe(3.5);
     histogram.labelValues("/world").observe(3.5);
 
@@ -1072,7 +1094,8 @@ public void markCurrentSpanAsExemplar() {}
                     "span_id",
                     "spanId-11"))
             .build();
-    Thread.sleep(sampleIntervalMillis + 1);
+    mockTime.advance(sampleIntervalMillis + 1);
+    Thread.sleep(50);
     histogram
         .labelValues("/hello")
         .observeWithExemplar(3.4, Labels.of("key1", "value1", "key2", "value2"));