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POgetting utc timestamp using strftime()
 

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  1. POgetting utc timestamp using strftime()
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    <p>I am trying to encode current utc time into string using the <code>strftime</code>function:</p> <pre><code>time_t now; struct tm nowLocal; struct tm nowUtc; now = time(NULL); localtime_r(&amp;now, &amp;nowLocal); gmtime_r(&amp;now, &amp;nowUtc); </code></pre> <p>So far so good: <code>nowLocal</code> contains current time in my timezone (CET), <code>nowUtc</code> contains utc time, the difference is exactly according to the <code>tm_gmtoff</code> value:</p> <pre><code>nowLocal: {tm_sec = 28, tm_min = 27, tm_hour = 13, tm_mday = 23, tm_mon = 4, tm_year = 112, tm_wday = 3, tm_yday = 143, tm_isdst = 1, tm_gmtoff = 7200, tm_zone = 0x8127a38 "CEST"} nowUtc: {tm_sec = 28, tm_min = 27, tm_hour = 11, tm_mday = 23, tm_mon = 4, tm_year = 112, tm_wday = 3, tm_yday = 143, tm_isdst = 0, tm_gmtoff = 0, tm_zone = 0x3e9907 "GMT"} </code></pre> <p>Then I call <code>strftime()</code> with <code>"%s"</code> format to get the seconds since epoch:</p> <pre><code>char tsFromLocal[32]; char tsFromUtc[32]; strftime(tsFromLocal, sizeof(tsFromLocal), "%s", &amp;nowLocal); strftime(tsFromUtc, sizeof(tsFromUtc), "%s", &amp;nowUtc); </code></pre> <p>The result seems strange to me. I expected to get exactly the same string from both <code>strftime()</code> calls as <code>%s</code> format is described as:</p> <p>The number of seconds since the epoch, i.e., since <code>1970-01-01 00:00:00 UTC</code>. Leap seconds are not counted unless leap second support is available. </p> <p>But I got two different values:</p> <pre><code>tsFromLocal:"1337772448" tsFromUtc: "1337768848" </code></pre> <p>and moreover the difference is not <strong><em>7200</em></strong> (<code>tm_gmtoff</code>) but <strong><em>3600</em></strong>. Can anyone explain such behavior? Or is it a bug?</p> <p>The reason why I am doing this is that I need to transfer the time value over the network and compare it to the current time on the target machine that can be in different time zone. On target machine I wanted to:</p> <pre><code>struct tm restoredUtc; time_t restored; strptime(tsFromUtc, "%s", &amp;restoredUtc); restored = timegm(&amp;restoredUtc); </code></pre> <p>But I got:</p> <pre><code>restoredUtc:{tm_sec = 28, tm_min = 27, tm_hour = 12, tm_mday = 23, tm_mon = 4, tm_year = 112, tm_wday = 3, tm_yday = 143, tm_isdst = 1, tm_gmtoff = 7200, tm_zone = 0x8127a38 "CEST"} </code></pre> <p>So <code>strptime()</code> sets the <code>tm_zone</code> according to the current time zone anyway. But even if I would use <code>timelocal()</code> instead of <code>timegm()</code> I will not get correct value as it should be <strong><em>11:27:28 CEST</em></strong> and not <strong><em>12:27:28 CEST</em></strong>. Is this error related to different results of <code>strftime()</code>?</p> <p>Any comments to this later part?</p>
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    1. COAs far as I know, `strftime()` interprets the passed time structure as LOCAL TIME. You are getting a difference of 1 hour because the original `nowLocal` structure has `tm_isdst = 1` (set by the previous call to localtime_r()). If you set this field to 0 before calling strftime, you will probably get the difference of 2 hours you expect.
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    2. COGiven that you have the time in seconds from the epoch in the result from `time()`, you can compare that with what `strftime()` returns with a view to working out which of the answers is correct (if either). My guess is that the local time is correct and the UTC is wrong. Whether you can then make `strftime()` work as you want short of setting the TZ variable to `UTC0` and then invoking `tzset()` is open for discussion.
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      1. USJonathan Leffler
 

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