actually, no it isn't
year-month-day sequence is mandatory for specifying a date string in query syntax (insert values, comparisons, etc.), but dates are stored in a completely different format
without any issue in correctness, yes, but definitely there is a deleterious issue in performance
the only way to allow an index on a column to be used, is to specify all conditions with the column on one side of a comparison operator
e.g. for january 2012 events, the only way to have the index on the date column to be used, for optimum efficiency, is...
WHERE datecol >= '2012-01-01'
AND datecol < '2012-02-01'