diff --git a/src/Cluster/Builder.c b/src/Cluster/Builder.c
index 7750f555b..acfa5c7cd 100644
--- a/src/Cluster/Builder.c
+++ b/src/Cluster/Builder.c
@@ -106,6 +106,27 @@ static zend_always_inline void php_scylladb_set_timeout(INTERNAL_FUNCTION_PARAME
     RETURN_ZVAL(getThis(), 1, 0);
 }
 
+static zend_always_inline void php_scylladb_set_interval_seconds(INTERNAL_FUNCTION_PARAMETERS, uint32_t *out_seconds)
+{
+    double interval = 0;
+
+    ZEND_PARSE_PARAMETERS_START(1, 1)
+    Z_PARAM_DOUBLE(interval)
+    ZEND_PARSE_PARAMETERS_END();
+
+    if (interval < 0)
+    {
+        zval val;
+        ZVAL_DOUBLE(&val, interval);
+        throw_invalid_argument(&val, "interval", "a positive number");
+        return;
+    }
+
+    *out_seconds = (uint32_t)ceil(interval);
+
+    RETURN_ZVAL(getThis(), 1, 0);
+}
+
 ZEND_METHOD(Cassandra_Cluster_Builder, build)
 {
 #ifndef PHP_SCYLLADB_BACKEND_SCYLLA_RUST
@@ -668,7 +689,7 @@ ZEND_METHOD(Cassandra_Cluster_Builder, withTCPKeepalive)
     }
 
     self->enable_tcp_keepalive = cass_true;
-    self->tcp_keepalive_delay = (uint32_t)ceil(delay * 1000);
+    self->tcp_keepalive_delay = (uint32_t)ceil(delay);
 
     RETURN_ZVAL(getThis(), 1, 0);
 }
@@ -760,5 +781,5 @@ ZEND_METHOD(Cassandra_Cluster_Builder, withRandomizedContactPoints)
 ZEND_METHOD(Cassandra_Cluster_Builder, withConnectionHeartbeatInterval)
 {
     auto self = PHP_SCYLLADB_GET_CLUSTER_BUILDER(getThis());
-    php_scylladb_set_timeout(INTERNAL_FUNCTION_PARAM_PASSTHRU, &self->connection_heartbeat_interval);
+    php_scylladb_set_interval_seconds(INTERNAL_FUNCTION_PARAM_PASSTHRU, &self->connection_heartbeat_interval);
 }
diff --git a/src/Cluster/BuilderHandlers.c b/src/Cluster/BuilderHandlers.c
index 2cda1c609..b405d1231 100644
--- a/src/Cluster/BuilderHandlers.c
+++ b/src/Cluster/BuilderHandlers.c
@@ -125,7 +125,7 @@ HashTable *php_scylladb_cluster_builder_properties(zend_object *object)
 
     if (self->enable_tcp_keepalive)
     {
-        ZVAL_DOUBLE(&tcpKeepalive, (double)self->tcp_keepalive_delay / 1000);
+        ZVAL_DOUBLE(&tcpKeepalive, (double)self->tcp_keepalive_delay);
     }
     else
     {
diff --git a/tests/Unit/Cluster/BuilderTest.php b/tests/Unit/Cluster/BuilderTest.php
new file mode 100644
index 000000000..b43838d51
--- /dev/null
+++ b/tests/Unit/Cluster/BuilderTest.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Cassandra\Tests\Unit\Cluster;
+
+use Cassandra;
+
+describe('Cassandra\Cluster\Builder', function () {
+
+    it('defaults connectionHeartbeatInterval to 30 seconds', function () {
+        $props = (array) Cassandra::cluster();
+
+        expect($props['connectionHeartbeatInterval'])->toBe(30);
+    });
+
+    it('defaults tcpKeepalive to null', function () {
+        $props = (array) Cassandra::cluster();
+
+        expect($props['tcpKeepalive'])->toBeNull();
+    });
+
+    it('round-trips connectionHeartbeatInterval in seconds', function (float $interval, int $expected) {
+        $props = (array) Cassandra::cluster()->withConnectionHeartbeatInterval($interval);
+
+        expect($props['connectionHeartbeatInterval'])->toBe($expected);
+    })->with([
+        [0.0, 0],
+        [5.0, 5],
+        [30.0, 30],
+        [3600.0, 3600],
+    ]);
+
+    it('round-trips tcpKeepalive in seconds', function (float $delay, float $expected) {
+        $props = (array) Cassandra::cluster()->withTCPKeepalive($delay);
+
+        expect($props['tcpKeepalive'])->toBe($expected);
+    })->with([
+        [0.0, 0.0],
+        [60.0, 60.0],
+        [1800.0, 1800.0],
+    ]);
+
+    it('disables tcpKeepalive when given null', function () {
+        $props = (array) Cassandra::cluster()->withTCPKeepalive(null);
+
+        expect($props['tcpKeepalive'])->toBeNull();
+    });
+
+    it('round-trips reconnectInterval in seconds', function () {
+        $props = (array) Cassandra::cluster()->withReconnectInterval(2.5);
+
+        expect($props['reconnectInterval'])->toBe(2.5);
+    });
+
+    it('rejects a negative connection heartbeat interval', function () {
+        Cassandra::cluster()->withConnectionHeartbeatInterval(-1.0);
+    })->throws(\Cassandra\Exception\InvalidArgumentException::class);
+
+    it('rejects a negative tcp keepalive delay', function () {
+        Cassandra::cluster()->withTCPKeepalive(-1.0);
+    })->throws(\Cassandra\Exception\InvalidArgumentException::class);
+});