Checks whether the dates are in a standard format and
sensible. If so, function returns the dates converted to ISO 8601
yyyy-mm-dd format. Acceptable formats are yyyy-mm-dd, yyyy/mm/dd,
mm/dd/yyyy, and dd/mm/yyyy. If format is mm/dd/yyyy or dd/mm/yyyy, the
dates must be interpretable unambiguously. Also, dates must be later than
1900-01-01. This additional check provides some protection against common
data entry errors like entering a year as 0217 or 1017 instead of 2017.