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    <p>The guy's asking for the <em>simplest</em> way. All your ways are unnecessarily complex. Just use modular arithmetic:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;time.h&gt; #define TARGET_TIME 21*60 // 9:00 PM, in format HH * 60 + MM int main(int argc, char* argv[]) { struct tm* tm; time_t ts = time(NULL); int minutes_until_9pm; tm = localtime(&amp;ts); minutes_until_9pm = 24*60 - (tm-&gt;tm_min + tm-&gt;tm_hour * 60 + TARGET_TIME)%(24*60); //alternatively, the following is slightly faster //minutes_until_9pm = 24*60 - (tm-&gt;tm_min + tm-&gt;tm_hour * 60 + TARGET_TIME) + (tm-&gt;tm_min+tm-&gt;tm_hour*60+TARGET_TIME&gt;24*60?24*60:0); printf("%d minutes til (next)9pm\n", minutes_until_9pm); return 0; } </code></pre> <p>If you know your timezone , it all becomes even simpler:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;time.h&gt; #define TARGET_TIME 21*60 // 9:00 PM, in format HH * 60 + MM #define TIMEZONE (-6) // This is CST; replace with your own timezone int main(int argc, char* argv[]) { int minutes_until_9pm = 24*60 - (time(NULL)/60 + TARGET_TIME + (24+TIMEZONE)*60)%(24*60); printf("%d minutes til (next)9pm\n", minutes_until_9pm); return 0; } </code></pre> <p>What's going on here is that, modulo one day, we're getting number of minutes since midnight. Then we're adding 9:00 PM to get the the number of minutes since 9:00 PM. Then we're subtracting that from 24 hours to get the number of minutes until the next 9:00 PM.</p> <p>If you need higher accuracy (i.e. rounded to the nearest number of minutes, rather than simply number of minutes), simply subtract <code>(tm-&gt;tm_sec&gt;=30?0:1)</code> from <code>minutes_until_9pm</code>, or replace <code>time(NULL)/60</code> with <code>(time(NULL)+30)/60</code>, respectively.</p>
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    1. POCalculate minutes until specific time? (C)
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    1. COReally simpler...sadly "time(NULL)/60" would offer 59 chances to get bad result falling into truncation of result. Moreover, have you ever looked for asm code of remainder by (21*60) ??!?! A sum if negative would be far easier to poor compiler, and by far more portable on **any** C flavor.
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    2. COWell, you act like the other solutions don't have the truncation problem. They do. I've added instructions for getting them to round, but in general, it's not that big of a deal. `(21*60)` was a typo, I meant `(24*60)`. Sure, the performance isn't great, but it's no worse than any other method of calculation, such as `localtime()`, which is what others are using. I'll optimize the first one nod to use modular arithmetic, though.
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    3. COFurthermore, considering `localtime()` is slower than a single modulus, I doubt the difference is meaningful.
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