title: Converting a DateTime Index value to an Index Number
body: <p>If I have a dataframe that has a datetime index and I get the first valid index by using series.first_valid_index - It returns a the date time of the first non nan which is what I'm looking for however: </p>
  
  <p>Is there a way to get the index number that the datetime value corresponds to. For example, it returns 2018-07-16 but I'd like to know that's the 18th row of the dataframe?</p>
  
  <p>If not, is there a way to count the rows from the beginning of the dataframe to that index value?</p>