[android] sync date with server date

android/nativebrik/src/main/java/com/nativebrik/sdk/data/experiment.kt

@@ -18,7 +18,7 @@ internal class ExperimentRepositoryImpl(private val config: Config): ExperimentR
     ): Result<ExperimentConfigs> {
         return withContext(Dispatchers.IO) {
             val url = config.endpoint.cdn + "/projects/" + config.projectId + "/experiments/id/" + id
-            val response: String = getRequest(url).getOrElse {
+            val response: String = getRequest(url, syncDateTime = true).getOrElse {
                 return@withContext Result.failure(it)
             }
             val json = Json.decodeFromString<JsonElement>(response)
@@ -30,7 +30,7 @@ internal class ExperimentRepositoryImpl(private val config: Config): ExperimentR
     override suspend fun fetchTriggerExperimentConfigs(name: String): Result<ExperimentConfigs> {
         return withContext(Dispatchers.IO) {
             val url = config.endpoint.cdn + "/projects/" + config.projectId + "/experiments/trigger/" + name
-            val response: String = getRequest(url).getOrElse {
+            val response: String = getRequest(url, syncDateTime = true).getOrElse {
                 return@withContext Result.failure(it)
             }
             val json = Json.decodeFromString<JsonElement>(response)

android/nativebrik/src/main/java/com/nativebrik/sdk/data/network.kt

@@ -1,5 +1,6 @@
 package com.nativebrik.sdk.data
 
+import com.nativebrik.sdk.data.user.syncDateFromHttpResponse
 import java.io.BufferedReader
 import java.io.IOException
 import java.io.InputStreamReader
@@ -9,9 +10,10 @@ import java.net.URL
 internal const val CONNECT_TIMEOUT = 10 * 1000
 internal const val READ_TIMEOUT = 5 * 1000
 
-internal fun getRequest(endpoint: String): Result<String> {
+internal fun getRequest(endpoint: String, syncDateTime: Boolean = false): Result<String> {
     var connection: HttpURLConnection? = null
     try {
+        val t0 = System.currentTimeMillis()
         val url = URL(endpoint)
         connection = url.openConnection() as HttpURLConnection
         connection.connectTimeout = CONNECT_TIMEOUT
@@ -23,6 +25,10 @@ internal fun getRequest(endpoint: String): Result<String> {
         connection.connect()
         val responseCode = connection.responseCode
 
+        if (syncDateTime) {
+            syncDateFromHttpResponse(t0, connection)
+        }
+
         if (responseCode == HttpURLConnection.HTTP_OK) {
             val sb = StringBuilder()
             var line: String?

android/nativebrik/src/main/java/com/nativebrik/sdk/data/user/user.kt

@@ -5,10 +5,15 @@ import android.content.SharedPreferences
 import android.os.Build
 import com.nativebrik.sdk.VERSION
 import com.nativebrik.sdk.schema.BuiltinUserProperty
+import java.net.HttpURLConnection
+import java.text.SimpleDateFormat
+import java.time.Instant
+import java.time.ZoneId
 import java.time.ZonedDateTime
 import java.time.format.DateTimeFormatter
 import java.time.temporal.ChronoUnit
 import java.util.Locale
+import java.util.TimeZone
 import java.util.UUID
 import kotlin.random.Random
 
@@ -22,8 +27,32 @@ internal fun getNativebrikUserSharedPreferences(context: Context): SharedPrefere
 internal const val USER_SEED_MAX = 100000000
 internal const val USER_SEED_KEY = "NATIVEBRIK_USER_SEED"
 
+internal var DATETIME_OFFSET: Long = 0
 internal fun getCurrentDate(): ZonedDateTime {
-    return ZonedDateTime.now()
+    val currentMillis = ZonedDateTime.now().toInstant().toEpochMilli()
+    return ZonedDateTime.ofInstant(
+        Instant.ofEpochMilli(currentMillis + DATETIME_OFFSET),
+        ZoneId.systemDefault()
+    )
+}
+
+internal fun syncDateFromHttpResponse(t0: Long, connection: HttpURLConnection) {
+    val t1 = System.currentTimeMillis()
+
+    val serverDateStr = connection.headerFields["Date"]?.firstOrNull() ?: return
+
+    val serverTime = try {
+        val formatter = SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss zzz", Locale.US)
+        formatter.timeZone = TimeZone.getTimeZone("GMT")
+        formatter.parse(serverDateStr)?.time ?: return
+    } catch (e: Exception) {
+        return
+    }
+
+    val networkDelay = (t1 - t0) / 2
+    val estimatedServerTime = serverTime + networkDelay
+
+    DATETIME_OFFSET = estimatedServerTime - t1
 }
 
 internal fun getToday(): ZonedDateTime {