title: what does this cronjob line mean
body: <p>I have a cron expression, which is somewhat different from the expressions I have worked before, specially because of the <code>?</code> operator( i don't know why it has been used)</p>
  
  <p>following is the format that i have been refering to as the standard format. but the expression i have given after this standard format is not following this structure. 
  could anybody help me to understand this new format. thanks in advance for any help.</p>
  
  <pre><code>* * * * * command to be executed
  - - - - -
  | | | | |
  | | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
  | | | ------- Month (1 - 12)
  | | --------- Day of month (1 - 31)
  | ----------- Hour (0 - 23)
  ------------- Minute (0 - 59)
  </code></pre>
  
  <p>expression : </p>
  
  <pre><code>0  15 15 ? * 1-7
  </code></pre>