diff --git a/runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java b/runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java
index 9a8ec278e8ee..a73dd521f864 100644
--- a/runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java
+++ b/runners/core-java/src/main/java/org/apache/beam/runners/core/SimpleDoFnRunner.java
@@ -58,6 +58,7 @@
 import org.apache.beam.sdk.values.Row;
 import org.apache.beam.sdk.values.TupleTag;
 import org.apache.beam.sdk.values.WindowingStrategy;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.FluentIterable;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Iterables;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Sets;
@@ -197,28 +198,10 @@ public <KeyT> void onTimer(
       Instant timestamp,
       Instant outputTimestamp,
       TimeDomain timeDomain) {
-
-    // The effective timestamp is when derived elements will have their timestamp set, if not
-    // otherwise specified. If this is an event time timer, then they have the timer's output
-    // timestamp. Otherwise, they are set to the input timestamp, which is by definition
-    // non-late.
-    Instant effectiveTimestamp;
-    switch (timeDomain) {
-      case EVENT_TIME:
-        effectiveTimestamp = outputTimestamp;
-        break;
-      case PROCESSING_TIME:
-      case SYNCHRONIZED_PROCESSING_TIME:
-        effectiveTimestamp = stepContext.timerInternals().currentInputWatermarkTime();
-        break;
-
-      default:
-        throw new IllegalArgumentException(String.format("Unknown time domain: %s", timeDomain));
-    }
+    Preconditions.checkNotNull(outputTimestamp, "outputTimestamp");
 
     OnTimerArgumentProvider<KeyT> argumentProvider =
-        new OnTimerArgumentProvider<>(
-            timerId, key, window, timestamp, effectiveTimestamp, timeDomain);
+        new OnTimerArgumentProvider<>(timerId, key, window, timestamp, outputTimestamp, timeDomain);
     invoker.invokeOnTimer(timerId, timerFamilyId, argumentProvider);
   }
 
@@ -1217,7 +1200,7 @@ public Timer align(Duration period) {
     }
 
     /**
-     * For event time timers the target time should be prior to window GC time. So it return
+     * For event time timers the target time should be prior to window GC time. So it returns
      * min(time to set, GC Time of window).
      */
     private Instant minTargetAndGcTime(Instant target) {
@@ -1237,13 +1220,12 @@ public Timer withOutputTimestamp(Instant outputTimestamp) {
     }
 
     /**
-     *
-     *
-     * <ul>
-     *   Ensures that:
-     *   <li>Users can't set {@code outputTimestamp} for processing time timers.
-     *   <li>Event time timers' {@code outputTimestamp} is set before window expiration.
-     * </ul>
+     * Ensures that a timer's {@code outputTimestamp} is set at or after the current input timestamp
+     * (minus allowed timestamp skew if set) and before the max timestamp of the window (plus
+     * allowed lateness). <br>
+     * If the outputTimestamp is not set, it is defaulted to either:
+     * <li>The firing timestamp for timers in the {@link TimeDomain#EVENT_TIME}
+     * <li>The current element timestamp for other time domains.
      */
     private void setAndVerifyOutputTimestamp() {
       if (outputTimestamp != null) {
@@ -1261,7 +1243,7 @@ private void setAndVerifyOutputTimestamp() {
                       + "earlier than the timestamp of the current input or timer (%s) minus the "
                       + "allowed skew (%s) and no later than %s. See the "
                       + "DoFn#getAllowedTimestampSkew() Javadoc for details on changing the "
-                      + "allowed skew.details on changing the allowed skew.",
+                      + "allowed skew.",
                   outputTimestamp,
                   elementInputTimestamp,
                   fn.getAllowedTimestampSkew().getMillis() >= Integer.MAX_VALUE
@@ -1269,22 +1251,18 @@ private void setAndVerifyOutputTimestamp() {
                       : PeriodFormat.getDefault().print(fn.getAllowedTimestampSkew().toPeriod()),
                   BoundedWindow.TIMESTAMP_MAX_VALUE));
         }
-      }
-
-      // Output timestamp is set to the delivery time if not initialized by an user.
-      if (outputTimestamp == null && TimeDomain.EVENT_TIME.equals(spec.getTimeDomain())) {
+      } else if (TimeDomain.EVENT_TIME.equals(spec.getTimeDomain())) {
+        // The outputTimestamp was unset and this is a timer in the EVENT_TIME domain. The output
+        // timestamp will be the firing timestamp.
         outputTimestamp = target;
-      }
-      // For processing timers
-      if (outputTimestamp == null) {
-        // For processing timers output timestamp will be:
-        // 1) timestamp of input element
-        // OR
-        // 2) output timestamp of firing timer.
+      } else {
+        // The outputTimestamp was unset and this is a timer in the PROCESSING_TIME
+        // (or SYNCHRONIZED_PROCESSING_TIME) domain. The output timestamp will be the timestamp of
+        // the element (or timer) setting this timer.
         outputTimestamp = elementInputTimestamp;
       }
 
-      Instant windowExpiry = window.maxTimestamp().plus(allowedLateness);
+      Instant windowExpiry = LateDataUtils.garbageCollectionTime(window, allowedLateness);
       if (TimeDomain.EVENT_TIME.equals(spec.getTimeDomain())) {
         checkArgument(
             !outputTimestamp.isAfter(windowExpiry),
diff --git a/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java b/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java
index a8559983de84..bc82b7be42b7 100644
--- a/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java
+++ b/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java
@@ -1025,13 +1025,26 @@ protected void fireTimer(TimerData timerData) {
     checkArgument(namespace instanceof WindowNamespace);
     BoundedWindow window = ((WindowNamespace) namespace).getWindow();
     timerInternals.onFiredOrDeletedTimer(timerData);
+    Instant effectiveOutputTimestamp;
+
+    if (timerData.getDomain() == TimeDomain.EVENT_TIME) {
+      effectiveOutputTimestamp = timerData.getOutputTimestamp();
+    } else {
+      // Flink does not set a watermark hold for the timer's output timestamp, and previous to
+      // https://github.com/apache/beam/pull/17262 processing time timers did not correctly emit
+      // elements at their output timestamp.  In this case we need to continue doing the wrong thing
+      // and using the output watermark rather than the firing timestamp.  Once flink correctly sets
+      // a  watermark hold for the output timestamp, this should be changed back.
+      effectiveOutputTimestamp = timerInternals.currentOutputWatermarkTime();
+    }
+
     pushbackDoFnRunner.onTimer(
         timerData.getTimerId(),
         timerData.getTimerFamilyId(),
         keyedStateInternals.getKey(),
         window,
         timerData.getTimestamp(),
-        timerData.getOutputTimestamp(),
+        effectiveOutputTimestamp,
         timerData.getDomain());
   }