diff --git a/app/src/main/java/ovh/smayzy/i2d_project_15_app/MainActivity.kt b/app/src/main/java/ovh/smayzy/i2d_project_15_app/MainActivity.kt
index b7dc77b..d74dd17 100644
--- a/app/src/main/java/ovh/smayzy/i2d_project_15_app/MainActivity.kt
+++ b/app/src/main/java/ovh/smayzy/i2d_project_15_app/MainActivity.kt
@@ -51,6 +51,8 @@ class MainActivity : AppCompatActivity() {
     private lateinit var tempCircle: TextView
     private lateinit var humCircle: TextView
 
+    private var startTime: Long = 0L
+
     private val isTesting = true
 
     override fun onCreate(savedInstanceState: Bundle?) {
@@ -139,6 +141,7 @@ class MainActivity : AppCompatActivity() {
             bluetoothSocket?.inputStream ?: return
         }
         val reader = BufferedReader(InputStreamReader(inputStream))
+        startTime = System.currentTimeMillis()
 
         while (true) {
             try {
@@ -152,7 +155,11 @@ class MainActivity : AppCompatActivity() {
                     val hum = parts[1].toFloatOrNull()
 
                     if (temp != null && hum != null) {
-                        val reading = SensorReading(System.currentTimeMillis(), temp, hum)
+                        val reading = SensorReading(
+                            secSinceStart = (System.currentTimeMillis() - startTime) / 1000,
+                            temperature = temp,
+                            humidity = hum
+                        )
                         sensorHistory.add(reading)
                         Log.d(tag, "Store: $reading")