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    <p>I would NOT use <code>mach_absolute_time()</code> because it queries a combination of the kernel and the processor for an absolute time using ticks (probably an uptime).</p> <p>What I would use: </p> <pre><code>CFAbsoluteTimeGetCurrent(); </code></pre> <p>This function is optimized to correct the difference in the iOS and OSX software and hardware.</p> <p><em><strong>Something Geekier</em></strong></p> <p>The quotient of a difference in <code>mach_absolute_time()</code> and <code>AFAbsoluteTimeGetCurrent()</code> is always around 24000011.154871</p> <p>Here is a log of my app:</p> <p><strong>Please note that final result time is a difference in <code>CFAbsoluteTimeGetCurrent()</code>'s</strong></p> <pre><code> 2012-03-19 21:46:35.609 Rest Counter[3776:707] First Time: 353900795.609040 2012-03-19 21:46:36.360 Rest Counter[3776:707] Second Time: 353900796.360177 2012-03-19 21:46:36.361 Rest Counter[3776:707] Final Result Time (difference): 0.751137 2012-03-19 21:46:36.363 Rest Counter[3776:707] Mach absolute time: 18027372 2012-03-19 21:46:36.365 Rest Counter[3776:707] Mach absolute time/final time: 24000113.153295 2012-03-19 21:46:36.367 Rest Counter[3776:707] ----------------------------------------------------- 2012-03-19 21:46:43.074 Rest Counter[3776:707] First Time: 353900803.074637 2012-03-19 21:46:43.170 Rest Counter[3776:707] Second Time: 353900803.170256 2012-03-19 21:46:43.172 Rest Counter[3776:707] Final Result Time (difference): 0.095619 2012-03-19 21:46:43.173 Rest Counter[3776:707] Mach absolute time: 2294833 2012-03-19 21:46:43.175 Rest Counter[3776:707] Mach absolute time/final time: 23999753.727777 2012-03-19 21:46:43.177 Rest Counter[3776:707] ----------------------------------------------------- 2012-03-19 21:46:46.499 Rest Counter[3776:707] First Time: 353900806.499199 2012-03-19 21:46:55.017 Rest Counter[3776:707] Second Time: 353900815.016985 2012-03-19 21:46:55.018 Rest Counter[3776:707] Final Result Time (difference): 8.517786 2012-03-19 21:46:55.020 Rest Counter[3776:707] Mach absolute time: 204426836 2012-03-19 21:46:55.022 Rest Counter[3776:707] Mach absolute time/final time: 23999996.639500 2012-03-19 21:46:55.024 Rest Counter[3776:707] ----------------------------------------------------- </code></pre>
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    1. POHow can I get a precise time, for example in milliseconds in Objective-C?
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    1. USKristian Glass
    1. USNate Symer
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    1. COI ended up using `mach_absolute_time()` with a `mach_timebase_info_data` and then did `(long double)((mach_absolute_time()*time_base.numer)/((1000*1000)*time_base.denom));`. To get the mach timebase, you can do `(void)mach_timebase_info(&your_timebase);`
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    2. COCFAbsoluteTimeGetCurrent() is uptime....
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    3. COAccording to the docs for CFAbsoluteTimeGetCurrent(), "The system time may decrease due to synchronization with external time references or due to an explicit user change of the clock." I don't understand why anyone would want to use something like this to measure elapsed time, if it can go backwards.
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      1. USKristopher Johnson
 

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