title: Boto3 cron: Parameter ScheduleExpression is not valid
body: <p>I am trying to schedule event just after 15 minute of execution as:</p>
  
  <pre><code>client = boto3.client('events')
  d = datetime.now() + timedelta(minutes=40)
  cronJob = "cron(" + str(d.hour) + " " + str(d.minute) + " * * ? *)"
  client.put_rule(Name='extractData', ScheduleExpression=cronJob, State='ENABLED', Description='This is rule extracting flurry data')
  try:
      client.put_targets( Rule='extractData', Targets=[ { 'Id': '1', 'Arn': 'arn:aws:lambda:ap-southeast-1:381409677897:function:flurry_extractReportOnDemand' }])
  except:
      print("\n###################\n")
      print ("Could not schedule")
      print("\n###################\n") 
  </code></pre>
  
  <p>This is giving me error as</p>
  
  <pre><code>Parameter ScheduleExpression is not valid.
  </code></pre>
  
  <p>Any idea why ?</p>