Android: How to check if users date is valid

Question

I know there are a lot of questions asked that are related to this but they all seems not to solve my problem. I want to check if the date on the user's device is correct, start an activity but in a case where the date on the users device is wrong, it should show an error activity that asks the user to adjust their date just like how whatsapp implemented theirs..

Answer 1

You must have server's timestamps to determine if the time in device is fake. Npt pool is a free service to help you get true timestamps.
To use, device must ONLINE, you can not check without the Internet.
Copy class SntpClient to your project: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/net/SntpClient.java

Code:

SntpClient client = new SntpClient();
long now = -1;
if (client.requestTime("pool.ntp.org", TIME_OUT)) {              
      now = client.getNtpTime();
      if (Math.abs(now - System.currentTimeMillis())    >= ONE_DAY){ 
           // the device's time is wrong
           startErrorActivity();
           ...
      } else {
           // the different time lower than 1 day
           startNextActivity();
      }
} else {
      // something wrong, can't get server's time          
}

Don't forget add INTERNET permission to your manifest