position date improvements

please consider incorporating these minor updates for these common cases, without which it is missing many dates which are present on the page:

- handle common case of a year without a month
  - add 01-01 so Date.parse doesn't fail, yielding a valid date instead of nil

- look for dtstamp when dtend is missing, as is the case with "present"
  - in current companies, the end date is of class dtstamp instead of dtend