diff --git a/composeApp/src/androidMain/kotlin/com/example/casera/network/ApiClient.android.kt b/composeApp/src/androidMain/kotlin/com/example/casera/network/ApiClient.android.kt
index b09b039..afecc27 100644
--- a/composeApp/src/androidMain/kotlin/com/example/casera/network/ApiClient.android.kt
+++ b/composeApp/src/androidMain/kotlin/com/example/casera/network/ApiClient.android.kt
@@ -8,6 +8,7 @@ import io.ktor.client.plugins.logging.*
 import io.ktor.serialization.kotlinx.json.*
 import kotlinx.serialization.json.Json
 import java.util.Locale
+import java.util.TimeZone
 
 actual fun getLocalhostAddress(): String = "10.0.2.2"
 
@@ -15,6 +16,10 @@ actual fun getDeviceLanguage(): String {
     return Locale.getDefault().language
 }
 
+actual fun getDeviceTimezone(): String {
+    return TimeZone.getDefault().id
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(OkHttp) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }
diff --git a/composeApp/src/commonMain/kotlin/com/example/casera/network/ApiClient.kt b/composeApp/src/commonMain/kotlin/com/example/casera/network/ApiClient.kt
index 71282a1..27b1d21 100644
--- a/composeApp/src/commonMain/kotlin/com/example/casera/network/ApiClient.kt
+++ b/composeApp/src/commonMain/kotlin/com/example/casera/network/ApiClient.kt
@@ -16,6 +16,13 @@ expect fun createHttpClient(): HttpClient
  */
 expect fun getDeviceLanguage(): String
 
+/**
+ * Get the device's timezone identifier (e.g., "America/Los_Angeles", "Europe/London").
+ * This is used to set the X-Timezone header for API requests
+ * so the server can calculate overdue tasks correctly based on the user's local time.
+ */
+expect fun getDeviceTimezone(): String
+
 object ApiClient {
     val httpClient = createHttpClient()
 
diff --git a/composeApp/src/iosMain/kotlin/com/example/casera/network/ApiClient.ios.kt b/composeApp/src/iosMain/kotlin/com/example/casera/network/ApiClient.ios.kt
index 30a1e19..17cd496 100644
--- a/composeApp/src/iosMain/kotlin/com/example/casera/network/ApiClient.ios.kt
+++ b/composeApp/src/iosMain/kotlin/com/example/casera/network/ApiClient.ios.kt
@@ -8,6 +8,7 @@ import io.ktor.client.plugins.logging.*
 import io.ktor.serialization.kotlinx.json.*
 import kotlinx.serialization.json.Json
 import platform.Foundation.NSLocale
+import platform.Foundation.NSTimeZone
 import platform.Foundation.preferredLanguages
 
 actual fun getLocalhostAddress(): String = "127.0.0.1"
@@ -19,6 +20,10 @@ actual fun getDeviceLanguage(): String {
     return firstLanguage?.split("-")?.firstOrNull() ?: "en"
 }
 
+actual fun getDeviceTimezone(): String {
+    return NSTimeZone.localTimeZone.name
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(Darwin) {
         install(ContentNegotiation) {
@@ -36,6 +41,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
 
         engine {
diff --git a/composeApp/src/jsMain/kotlin/com/example/casera/network/ApiClient.js.kt b/composeApp/src/jsMain/kotlin/com/example/casera/network/ApiClient.js.kt
index 95f7aad..1e1504a 100644
--- a/composeApp/src/jsMain/kotlin/com/example/casera/network/ApiClient.js.kt
+++ b/composeApp/src/jsMain/kotlin/com/example/casera/network/ApiClient.js.kt
@@ -15,6 +15,11 @@ actual fun getDeviceLanguage(): String {
     return window.navigator.language.split("-").firstOrNull() ?: "en"
 }
 
+actual fun getDeviceTimezone(): String {
+    // Use Intl API to get IANA timezone name (e.g., "America/Los_Angeles")
+    return js("Intl.DateTimeFormat().resolvedOptions().timeZone") as String
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(Js) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }
diff --git a/composeApp/src/jvmMain/kotlin/com/example/casera/network/ApiClient.jvm.kt b/composeApp/src/jvmMain/kotlin/com/example/casera/network/ApiClient.jvm.kt
index f2fc711..eb5513c 100644
--- a/composeApp/src/jvmMain/kotlin/com/example/casera/network/ApiClient.jvm.kt
+++ b/composeApp/src/jvmMain/kotlin/com/example/casera/network/ApiClient.jvm.kt
@@ -8,6 +8,7 @@ import io.ktor.client.plugins.logging.*
 import io.ktor.serialization.kotlinx.json.*
 import kotlinx.serialization.json.Json
 import java.util.Locale
+import java.util.TimeZone
 
 actual fun getLocalhostAddress(): String = "127.0.0.1"
 
@@ -15,6 +16,10 @@ actual fun getDeviceLanguage(): String {
     return Locale.getDefault().language
 }
 
+actual fun getDeviceTimezone(): String {
+    return TimeZone.getDefault().id
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(CIO) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }
diff --git a/composeApp/src/wasmJsMain/kotlin/com/example/casera/network/ApiClient.wasmJs.kt b/composeApp/src/wasmJsMain/kotlin/com/example/casera/network/ApiClient.wasmJs.kt
index 95f7aad..1e1504a 100644
--- a/composeApp/src/wasmJsMain/kotlin/com/example/casera/network/ApiClient.wasmJs.kt
+++ b/composeApp/src/wasmJsMain/kotlin/com/example/casera/network/ApiClient.wasmJs.kt
@@ -15,6 +15,11 @@ actual fun getDeviceLanguage(): String {
     return window.navigator.language.split("-").firstOrNull() ?: "en"
 }
 
+actual fun getDeviceTimezone(): String {
+    // Use Intl API to get IANA timezone name (e.g., "America/Los_Angeles")
+    return js("Intl.DateTimeFormat().resolvedOptions().timeZone") as String
+}
+
 actual fun createHttpClient(): HttpClient {
     return HttpClient(Js) {
         install(ContentNegotiation) {
@@ -32,6 +37,7 @@ actual fun createHttpClient(): HttpClient {
 
         install(DefaultRequest) {
             headers.append("Accept-Language", getDeviceLanguage())
+            headers.append("X-Timezone", getDeviceTimezone())
         }
     }
 }