Thread: How can i store time in a variable for comparsion?

I'm not sure how to store time in a variable without it having the value of current run time
I've never done this before.

scenario
( don't look to much into the scenario as it's the theory of storing time I'm interested in learning. )
(Id have the same question about timers too)

I have a thread the thread sleeps for a time period called thread_sleep_time
I created one more variable called thread_total_sleep_time that is thread_total_sleep_time += thread_sleep_time, every time the thread is sleeps.

but how can I capture store a time in a variable called previous_time ? must I use keyword final so it never updates ?
for comparing against the current time.

Thank you

I'm not sure I'm bit confused to be honest.
I need to do this sort of thing

if(thread_current_total_sleep_time == thread_previous_total_sleep_time + 2000000)
{
update_previous_and _current() // somthing like this.
}

or something similar i will know how much time has passed since a last interval update

I'm confused at what you're trying to do. Is it more than just the basic, checking system time? i.e.,

      long initialTime = System.currentTimeMillis();
      while (somethingIsTrue) {
 
          // .... do something
 
      } 
 
      long finalTime = System.currentTimeMillis();
 
      long mSecDiff = finalTime - initialTime;