title: daylight saving time influences the result of mktime
body: <p>I have an issue trying to get the exact GPS time. Looks like the issue is related to command <code>mktime</code> used in the code.
  To get the correct GPS time I have added -2 hrs in UTC.</p>
  
  <p>But the purpose is to have a code to calculate the GPS time automatically and don't use manually changes at UTC-2 as an example</p>
  
  <p><strong>Input file GMT time ( > 3 hrs in local time )</strong></p>
  
  <pre><code>18/05/21 08:18:29
  18/05/21 08:20:25
  18/05/21 08:21:02
  </code></pre>
  
  <p>When I run the code below, got 1 hour less :</p>
  
  <pre><code>gawk -F'[/: ]' -v ts=$(date -d'01/06/1980' +%s)  \
                  -v lap=18 '{$1="" d=sprintf(20$0); 
                              print mktime(d)+lap-ts }'  file
  </code></pre>
  
  <p><strong>result output GPS TIME ( &lt; 1 hr )</strong></p>
  
  <pre><code>1210922327
  1210922443
  1210922480
  </code></pre>
  
  <p>When I run the code below, got exactly the correct GPS time.</p>
  
  <pre><code>gawk -F'[/: ]' -v ts=$(TZ=UTC-2 date -d'1/6/1980 0:00' +%s)  \
                  -v lap=18 '{$1="" d=sprintf(20$0); 
                              print mktime(d)+lap-ts }' file
  </code></pre>
  
  <p><strong>Result output GPS TIME ( exactly time: OUTPUT DESIRED)</strong></p>
  
  <pre><code>1210925927
  1210926043
  1210926080
  </code></pre>
  
  <p>Thanks in advance</p>