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    <p>The simplest way is to use 64-bit integer type, but I assume you cannot do this. Since you want your answer in 32-bit integer, the high-order value of microseconds cannot be greater than 999, or it would not fit in 32-bit after division by 1000. So the bigger number of microseconds you're operating with is <code>999 * 2^32 + (2^32 - 1) = 4294967295999</code>. It gives you 13 decimal digits and you can use <code>double</code> to handle precise division.</p> <p>If you are forced for some reason to use only 32-bit integers, the answer of Michał Górny gives you an <em>approximate</em> solution. E.g. for <code>whole_number = 1234567890123</code> it will give a result of <code>1234567805</code>. Because dividing of max 32-bit int on 1000 have a reminder.</p> <p>The only way to have an exact answer with 32-bit integer is by using long arithmetic. It requires long digits to be stored in a type which can be extended to store a reminder. You have to split your two 32-bit integers in four 16-bit digits. After that you can divide it as on paper and you have enough bits to store a reminder. See the code of <code>micro2milli</code>:</p> <pre><code>#include &lt;iostream&gt; typedef unsigned __int32 uint32; typedef unsigned __int64 uint64; const uint32 MAX_INT = 0xFFFFFFFF; uint32 micro2milli(uint32 hi, uint32 lo) { if (hi &gt;= 1000) { throw std::runtime_error("Cannot store milliseconds in uint32!"); } uint32 r = (lo &gt;&gt; 16) + (hi &lt;&lt; 16); uint32 ans = r / 1000; r = ((r % 1000) &lt;&lt; 16) + (lo &amp; 0xFFFF); ans = (ans &lt;&lt; 16) + r / 1000; return ans; } uint32 micro2milli_simple(uint32 hi, uint32 lo) { lo /= 1000; return lo + (hi % 1000) * 4294967L; } void main() { uint64 micro = 1234567890123; uint32 micro_high = micro &gt;&gt; 32; uint32 micro_low = micro &amp; MAX_INT; // 1234567805 std::cout &lt;&lt; micro2milli_simple(micro_high, micro_low) &lt;&lt; std::endl; // 1234567890 std::cout &lt;&lt; micro2milli(micro_high, micro_low) &lt;&lt; std::endl; } </code></pre>
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    1. PODividing a 2x32 bit big integer by 1000
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    1. CO`main` should return `int`, not `void`.
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