title: Calculate number of minutes until next run
body: <p>This should be trivial, but my brain is running into a brick wall tonight.</p>
  
  <p>I have a process that will run once, then every <code>n</code> minutes of an hourly offset.</p>
  
  <p>EG, 15 minutes means run on the hour (say, 1), then 1:15, 1:30, 1:45, etc.</p>
  
  <p>If the execution interval is 15, the initial execution is at 1:07 and runs until 1:11, the wait for the next execution will be in 4 minutes.</p>
  
  <p>If the execution interval is 15, the initial execution is at 1:07 and runs until 1:16, the wait for the next execution will be 14 minutes (execution times do not overlap).</p>
  
  <p>So, after every run (of an varying length), how do we calculate the number of minutes to wait until the next execution?</p>
  
  <hr>
  
  <p>I'm not putting any sample code in here, because I don't have anything that will pass my unit tests (I'm not so brain-dead that I can't write <em>those</em> !).</p>
  
  <p>I've tagged this as <code>C#</code> since that's what I'm coding in, but I would expect the solution to be language-agnostic.</p>