Is it reasonable to use System.currentTimeMillis() in conjunction with System.nanoTime()?

Question

Here is my modest proposal:

When the JVM starts
1. Call System.currentTimeMillis() and store as starting "wall clock" time:
  long currentTimeMillis0.
2. Call System.nanoTime() and store as starting "nano" time:
  long nanoTime0.
Throughout the run of the program, store many timestamps using System.nanoTime().

I am well aware the exact value of a nanoTime is useless by itself, but useful when used in the context of a duration: endNanoTime - beginNanoTime.

Is it then reasonable, at any nanoTime recorded, to calculate "currentTimeNanos" as:
(1000L * currentTimeMillis0) + (anyNanoTime - nanoTime0)?

Yes... in a single JVM instance this will be a good enough. – gladiator Jan 10 '16 at 16:10 — gladiator, Jan 10 '16 at 16:10

score 1 · Answer 1 · answered Jan 10 '16 at 15:56

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I think you need to combine start/end times, both in millis and nano. For example:

long startTimeMillis = System.currentTimeMillis() * 1000;
long startTimeNano = System.nanoTime();
... // do some work
long currentTimeNano = (System.currentTimeMillis()*1000 - startTimeMillis) + (System.nanoTime() - startTimeNano);

For more information, the official javadoc of the nanoTime method: https://docs.oracle.com/javase/1.5.0/docs/api/java/lang/System.html#nanoTime()

answered Jan 10 '16 at 15:56

Alejandro Goñi

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It appears to be a complicated way to calculate the nano-time difference plus 0.1% with a slight error. Can you explain how this is better than nano time alone? – Peter Lawrey Jan 10 '16 at 16:13
The scale between milli and nano is 1000000 not 1000. But in Java you can't do anything useful with sub-milli instant (as opposed to duration) anyway. – dave_thompson_085 Jan 10 '16 at 17:52

score 1 · Accepted Answer · answered Jan 10 '16 at 16:16

Where I use milli-second and nano-time is I use nano-time for calculating small differences in time and a milli-second timer once at the start to give context.

Note: it is possible to use nano-time between machine if you have a continuous stream of timings. i.e. you can estimate the best timings between them and detect outliers (even if you don't know the absolute difference) I use this class to help estimate the RunningMinimum

Is it reasonable to use System.currentTimeMillis() in conjunction with System.nanoTime()?

2 Answers2