Date.prototype.getTime

P T Withington ptw at pobox.com
Wed Jun 28 13:39:16 PDT 2006


On 2006-06-28, at 15:47 EDT, Brendan Eich wrote:

>> Date.tick:  getter returning an integer that increases linearly  
>> with elapsed time and is guaranteed to increase by at least 1 when  
>> accessed successively.  (I.e., can be used to measure the elapsed  
>> time of a trivial expression.)
>>
>> Date.tickScale:  float that is the ratio of Date.tick units to  
>> nanoseconds.
>
> That's not bad, but why not just give 'em nanoseconds since the  
> epoch so they don't have to normalize.

a) For when nanoseconds is too coarse?  tickScale could be to any  
convenient unit, seconds would be fine too.

b) So the language implementor can choose a scale that fits their  
implementation:  fine enough to be able to time the simplest  
expression accurately, but not so fine as to cause huge overhead in  
the implementation.

I think the applications for Date and 'tick' are different.  Date  
needs to be relative to an epoch because people use it for absolute  
times.  'tick' does not.  It is for very accurate measurement of  
intervals.  It can start with an arbitrary value.  It can wrap.  From  
the size of integers and the scale, you can determine the maximum  
interval you can measure, use modular arithmetic, and use Date to  
compensate if you really need that granularity for a longer interval  
(I believe such applications will be rare).

I even think that having an accurate, fine-grained clock as a  
primitive is important enough that I would give up synchronizing it  
with Date for greater accuracy.  If computing [ milliseconds,  
nanoseconds ] takes more time that the expression I am trying to  
measure, I have a problem.

[In case you haven't guessed, I am writing a Javascript profiler in  
Javascript and am being thwarted by the lack of granularity in Date.   
And, I didn't invent the tick/scale interface.  I stole it from the  
DEC Alpha which has a control register that increments at the CPU  
clock frequency and a register that tells you what that frequency  
is.  This register is readable in user space, so makes a really fine  
profiling instrument.]



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