Project Url: lyft/Kronos-Android
Introduction: An Open Source Kotlin SNTP library
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Synchronized Time Android Library

Kronos is an open source Network Time Protocol (NTP) synchronization library for providing a trusted clock on the JVM.

Unlike the device clock, the time reported by Kronos is unaffected when the local time is changed while your app is running. Instead, Kronos stores accurate time along with a delta between the NTP time and the system uptime. Since uptime increases monotonically, Kronos isn't affected by device time changes. Accessing KronosClock.getCurrentTimeMs() will return the local time based on the last known accurate time + delta since last sync.


Include the following in your build.gradle file:

implementation "com.lyft.kronos:kronos-android:$latest_version"

Obtain a Kronos clock that is synchronized with NTP servers.

class YourApplication : Application() {

    lateinit var kronosClock: KronosClock

    override fun onCreate() {

        kronosClock = AndroidClockFactory.createKronosClock(applicationContext)

Replace usages of




If the NTP server cannot be reached or Kronos has not yet been synced, getCurrentTimeMs() will return time from the fallback clock and trigger syncInBackground(). If you'd rather control the fallback, you can use getCurrentNtpTimeMs(), which returns null instead of falling back. To get metadata with an individual timestamp, use KronosClock.getCurrentTime(), which returns an instance of KronosTime. KronosTime contains the currentTime and the timeSinceLastNtpSyncMs, which will be null if currentTime is coming from the device clock.

Since it relies on system uptime, Kronos detects and requires a new sync after each reboot.


Kronos comes with a set of reasonable default configurations. You can customize the configuration by using AndroidClockFactory.createKronosClock with the following optional parameters:

  • syncListener
    • Allows you to log sync operation successes and errors, which maybe useful for custom analytics. Pass an implementation of SyncListener.
  • ntpHosts
    • Specify a list of NTP servers with which to sync.
  • requestTimeoutMs
    • Lengthen or shorten the timeout value. If the NTP server fails to respond within the given time, the next server will be contacted. If none of the server respond within the given time, the sync operation will be considered a failure.
  • minWaitTimeBetweenSyncMs
    • Kronos attempts a synchronization at most once a minute. If you want to change the frequency, supply the desired interval in milliseconds. Note that you should also supply a cacheExpirationMs value. For example, if you shorten the minWaitTimeBetweenSyncMs to 30 seconds, but leave the cacheExpirationMs to 1 minute, it will have no affect because the cache is still valid within the 1 minute window.
  • cacheExpirationMs
    • Kronos will perform a background sync if the cache is stale. The cache is valid for 1 minute by default. It is simpliest to keep the cacheExpirationMs value the same as minWaitTimeBetweenSyncMs value.

With or without Android

For usage with non-Android modules, Kronos provides access to the Kotlin-only base library called Kronos-Java, which depends on an externally provided local clock and a cache. The Android library simply abstracts away the creation of the clock and cache by extracting the Android system clock from a provided Context and creating its own cache using SharedPreferences.

To use Kronos-Java include the following in your build.gradle file:

implementation "com.lyft.kronos:kronos-java:$latest_version"

Version infromation are listed under releases

Looking for Kronos for your iOS application? Check out Kronos for iOS


Copyright (C) 2018 Lyft Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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