title: Sequence of dates by each next 15th or last day of month
body: <p>I have start and end dates as e.g. 03-05-2014 and 01-07-2014. I would like to create a code so that it would output sequences such as next 15th of each month or next end day of the month, in this case 15-05-2014 &amp; 15-06-2014, or 31-05-2014 and 31-06-2014.</p>
  
  <p>I know this would be possible by creating sequence of days and then identifying the particular day (at least for the 1st case) as in <code>seq(1st date, 2nd date, by = "day")</code>, but let's just say that for computational limitations this is not possible - I have to create it for many years and millions of records which need to be grouped as well.</p>
  
  <p>Is there any workaround there?</p>