time since JVM started

Use this snippet:

long jvmUpTime = ManagementFactory.getRuntimeMXBean().getUptime();

or:

long jvmStartTime = ManagementFactory.getRuntimeMXBean().getStartTime();

This is the correct way of retrieving JVM up-time.

For more info see RuntimeMXBean

You can get the start time of the JVM in the following code:

import java.lang.management.ManagementFactory;
  import java.lang.management.RuntimeMXBean;
  ...
  RuntimeMXBean runtimeMXBean = ManagementFactory.getRuntimeMXBean();
  long uptimeInMillis = runtimeMXBean.getUptime();

See more are https://docs.oracle.com/javase/6/docs/api/java/lang/management/RuntimeMXBean.html.

if your jvm program running in linux, you can view the startTime use ps

ps -p <pid> -o stime,etime