So I want is to get the value of Firestore timestamp and Local device currentTime, and display the different:
Timestamp timestamp = (Timestamp) document.getData().get("createdAt");
Long tsLong = System.currentTimeMillis()/1000;
//I need the result value of (tsLong - timestamp seconds)
But I'm stuck at the value calculating, how is it possible to get the milliseconds of the timestamp to Long type?
In firestore timestamp, we have an API getSeconds(). This API return time in second (long data type).
Timestamp timestamp = (Timestamp) document.getData().get("createdAt");
Long tsLong = System.currentTimeMillis()/1000;
long result = tsLong - timestamp.getSeconds();
Do not perform the calculation yourself if there is a standard API available to do the same.
import java.time.Instant;
import java.util.concurrent.TimeUnit;
public class Main {
public static void main(String[] args) {
long seconds = TimeUnit.SECONDS.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS);
System.out.println(seconds);
// Alternatively, using java.time API
seconds = Instant.now().getEpochSecond();
System.out.println(seconds);
}
}
You can use DocumentSnapshot#getTimestamp to get the Timestamp from which you can get the seconds using Timestamp#getSeconds.
Thus, you can do it like
long diff = TimeUnit.SECONDS.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS) - document.getTimestamp("createdAt").getSeconds();
The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API*.
Solution using java.time, the modern Date-Time API:
Using Timestamp#todate, you can convert Timestamp to java.util.Date which can be converted into java.time.Instant and then you can use java.time.Duration to find the difference in seconds as shown below:
long seconds = Duration.between(Instant.now(), document.getTimestamp("createdAt").toDate().toInstant()).toSeconds();
Learn more about the modern Date-Time API from Trail: Date Time.
* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.
DocumentSnapshot class contains a method called getTimestamp(String field). So a simpler way to get the difference would be:
Timestamp createdAt = document.getTimestamp("createdAt");
Long seconds = System.currentTimeMillis()/1000;
long difference = seconds - createdAt.getSeconds();
