diff --git a/CHANGELOG.md b/CHANGELOG.md
index 17d3cac7..c25166fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Nylas Java SDK Changelog
 
+## [Unreleased]
+
+### Changed
+* `SendMessageRequest.sendAt` field changed from `Int?` to `Long?` to support Unix timestamps beyond 2038. Maintains backward compatibility through method overloading - existing `Int` parameters are automatically converted to `Long`. **Note:** Kotlin users passing integer literals may need to specify type explicitly (e.g., `1620000000L` or `1620000000 as Int`).
+
 ## [2.13.1]
 
 ### Fixed
diff --git a/src/main/kotlin/com/nylas/models/SendMessageRequest.kt b/src/main/kotlin/com/nylas/models/SendMessageRequest.kt
index c69f59f1..7366b1ab 100644
--- a/src/main/kotlin/com/nylas/models/SendMessageRequest.kt
+++ b/src/main/kotlin/com/nylas/models/SendMessageRequest.kt
@@ -56,7 +56,7 @@ data class SendMessageRequest(
    * Unix timestamp to send the message at.
    */
   @Json(name = "send_at")
-  val sendAt: Int? = null,
+  val sendAt: Long? = null,
   /**
    * The ID of the message that you are replying to.
    */
@@ -94,7 +94,7 @@ data class SendMessageRequest(
     internal var subject: String? = null
     private var body: String? = null
     private var starred: Boolean? = null
-    private var sendAt: Int? = null
+    private var sendAt: Long? = null
     private var replyToMessageId: String? = null
     private var trackingOptions: TrackingOptions? = null
     private var useDraft: Boolean? = null
@@ -162,7 +162,14 @@ data class SendMessageRequest(
      * @param sendAt The unix timestamp to send the message at.
      * @return The builder.
      */
-    fun sendAt(sendAt: Int?) = apply { this.sendAt = sendAt }
+    fun sendAt(sendAt: Long?) = apply { this.sendAt = sendAt }
+
+    /**
+     * Sets the unix timestamp to send the message at.
+     * @param sendAt The unix timestamp to send the message at (automatically converted to Long).
+     * @return The builder.
+     */
+    fun sendAt(sendAt: Int?) = apply { this.sendAt = sendAt?.toLong() }
 
     /**
      * Sets the ID of the message that you are replying to.
diff --git a/src/test/kotlin/com/nylas/resources/MessagesTests.kt b/src/test/kotlin/com/nylas/resources/MessagesTests.kt
index c1a66f16..0fd52dad 100644
--- a/src/test/kotlin/com/nylas/resources/MessagesTests.kt
+++ b/src/test/kotlin/com/nylas/resources/MessagesTests.kt
@@ -616,6 +616,75 @@ class MessagesTests {
       assertNull(queryParamCaptor.firstValue)
     }
 
+    @Test
+    fun `sending a message with Int sendAt value calls requests with the correct params`() {
+      val adapter = JsonHelper.moshi().adapter(SendMessageRequest::class.java)
+      val sendMessageRequest =
+        SendMessageRequest.Builder(to = listOf(EmailName(email = "test@gmail.com", name = "Test")))
+          .body("Hello, I just sent a message using Nylas!")
+          .subject("Hello from Nylas!")
+          .sendAt(1620000000 as Int) // Explicitly test Int overload conversion to Long
+          .build()
+
+      messages.send(grantId, sendMessageRequest)
+
+      val pathCaptor = argumentCaptor<String>()
+      val typeCaptor = argumentCaptor<Type>()
+      val requestBodyCaptor = argumentCaptor<String>()
+      val queryParamCaptor = argumentCaptor<IQueryParams>()
+      val overrideParamCaptor = argumentCaptor<RequestOverrides>()
+      verify(mockNylasClient).executePost<Response<Message>>(
+        pathCaptor.capture(),
+        typeCaptor.capture(),
+        requestBodyCaptor.capture(),
+        queryParamCaptor.capture(),
+        overrideParamCaptor.capture(),
+      )
+
+      assertEquals("v3/grants/$grantId/messages/send", pathCaptor.firstValue)
+      assertEquals(Types.newParameterizedType(Response::class.java, Message::class.java), typeCaptor.firstValue)
+      assertEquals(adapter.toJson(sendMessageRequest), requestBodyCaptor.firstValue)
+      assertNull(queryParamCaptor.firstValue)
+
+      // Verify the sendAt field is properly set as Long
+      assertEquals(1620000000L, sendMessageRequest.sendAt)
+    }
+
+    @Test
+    fun `sending a message with Long sendAt value calls requests with the correct params`() {
+      val adapter = JsonHelper.moshi().adapter(SendMessageRequest::class.java)
+      val longTimestamp = 1893456000L // Year 2030 timestamp that exceeds Int.MAX_VALUE in the future
+      val sendMessageRequest =
+        SendMessageRequest.Builder(to = listOf(EmailName(email = "test@gmail.com", name = "Test")))
+          .body("Hello, I just sent a message using Nylas!")
+          .subject("Hello from Nylas!")
+          .sendAt(longTimestamp) // Long value
+          .build()
+
+      messages.send(grantId, sendMessageRequest)
+
+      val pathCaptor = argumentCaptor<String>()
+      val typeCaptor = argumentCaptor<Type>()
+      val requestBodyCaptor = argumentCaptor<String>()
+      val queryParamCaptor = argumentCaptor<IQueryParams>()
+      val overrideParamCaptor = argumentCaptor<RequestOverrides>()
+      verify(mockNylasClient).executePost<Response<Message>>(
+        pathCaptor.capture(),
+        typeCaptor.capture(),
+        requestBodyCaptor.capture(),
+        queryParamCaptor.capture(),
+        overrideParamCaptor.capture(),
+      )
+
+      assertEquals("v3/grants/$grantId/messages/send", pathCaptor.firstValue)
+      assertEquals(Types.newParameterizedType(Response::class.java, Message::class.java), typeCaptor.firstValue)
+      assertEquals(adapter.toJson(sendMessageRequest), requestBodyCaptor.firstValue)
+      assertNull(queryParamCaptor.firstValue)
+
+      // Verify the sendAt field is properly set as Long
+      assertEquals(longTimestamp, sendMessageRequest.sendAt)
+    }
+
     @Test
     fun `sending a message with a large attachment calls requests with the correct params`() {
       val adapter = JsonHelper.moshi().adapter(SendMessageRequest::class.java)