diff --git a/src/lib/waku_relay/index.node.spec.ts b/src/lib/waku_relay/index.node.spec.ts
index f6d9b5ac47..b5840cfcb1 100644
--- a/src/lib/waku_relay/index.node.spec.ts
+++ b/src/lib/waku_relay/index.node.spec.ts
@@ -379,7 +379,7 @@ describe("Waku Relay [node only]", () => {
         }
       );
 
-      await nimWaku.sendMessage(message);
+      await nimWaku.sendMessage(NimWaku.toWakuRelayMessage(message));
 
       const receivedMsg = await receivedMsgPromise;
 
diff --git a/src/lib/waku_store/index.node.spec.ts b/src/lib/waku_store/index.node.spec.ts
index dd6b148782..e0d42c7d19 100644
--- a/src/lib/waku_store/index.node.spec.ts
+++ b/src/lib/waku_store/index.node.spec.ts
@@ -40,7 +40,9 @@ describe("Waku Store", () => {
     for (let i = 0; i < 2; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          )
         )
       ).to.be.true;
     }
@@ -70,7 +72,9 @@ describe("Waku Store", () => {
     for (let i = 0; i < totalMsgs; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          )
         )
       ).to.be.true;
     }
@@ -107,7 +111,9 @@ describe("Waku Store", () => {
     for (let i = 0; i < availMsgs; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          )
         )
       ).to.be.true;
     }
@@ -141,7 +147,9 @@ describe("Waku Store", () => {
     for (let i = 0; i < 15; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          )
         )
       ).to.be.true;
     }
@@ -176,7 +184,9 @@ describe("Waku Store", () => {
     for (let i = 0; i < 2; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic),
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic)
+          ),
           customPubSubTopic
         )
       ).to.be.true;
@@ -453,9 +463,11 @@ describe("Waku Store", () => {
     for (let i = 0; i < 2; i++) {
       expect(
         await nimWaku.sendMessage(
-          await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic, {
-            timestamp: messageTimestamps[i],
-          })
+          NimWaku.toWakuRelayMessage(
+            await WakuMessage.fromUtf8String(`Message ${i}`, TestContentTopic, {
+              timestamp: messageTimestamps[i],
+            })
+          )
         )
       ).to.be.true;
       if (!firstMessageTime) firstMessageTime = Date.now() / 1000;
diff --git a/src/test_utils/nim_waku.ts b/src/test_utils/nim_waku.ts
index 608beed562..0894916841 100644
--- a/src/test_utils/nim_waku.ts
+++ b/src/test_utils/nim_waku.ts
@@ -62,9 +62,9 @@ export interface KeyPair {
 }
 
 export interface WakuRelayMessage {
-  payload: string;
+  payload: string; // Hex encoded data string without `0x` prefix.
   contentTopic?: string;
-  timestamp?: number; // Integer in seconds
+  timestamp?: number; // Unix epoch time in nanoseconds as a 64-bits integer value.
 }
 
 export class NimWaku {
@@ -75,6 +75,27 @@ export class NimWaku {
   private readonly logPath: string;
   private rpcPort?: number;
 
+  /**
+   * Convert a [[WakuMessage]] to a [[WakuRelayMessage]]. The latter is used
+   * by the nwaku JSON-RPC API.
+   */
+  static toWakuRelayMessage(message: WakuMessage): WakuRelayMessage {
+    if (!message.payload) {
+      throw "Attempting to convert empty message";
+    }
+
+    let timestamp;
+    if (message.proto.timestamp) {
+      timestamp = message.proto.timestamp.toNumber();
+    }
+
+    return {
+      payload: bytesToHex(message.payload),
+      contentTopic: message.contentTopic,
+      timestamp,
+    };
+  }
+
   constructor(logName: string) {
     this.logPath = `${LOG_DIR}/nim-waku_${logName}.log`;
   }
@@ -184,28 +205,14 @@ export class NimWaku {
   }
 
   async sendMessage(
-    message: WakuMessage,
+    message: WakuRelayMessage,
     pubSubTopic?: string
   ): Promise<boolean> {
     this.checkProcess();
 
-    if (!message.payload) {
-      throw "Attempting to send empty message";
-    }
-    let timestamp;
-    if (message.timestamp) {
-      timestamp = message.timestamp.valueOf() * 1000;
-    }
-
-    const rpcMessage = {
-      payload: bytesToHex(message.payload),
-      contentTopic: message.contentTopic,
-      timestamp,
-    };
-
     return this.rpcCall<boolean>("post_waku_v2_relay_v1_message", [
       pubSubTopic ? pubSubTopic : DefaultPubSubTopic,
-      rpcMessage,
+      message,
     ]);
   }