Returns true if str is a date.
isDate(str, [format])
str string, string to format
format string, optional, format to check
You can specify date format. The format can contain following elements: mm, dd, yyyy, mon in any order with any formatting characters. For example 'mm.dd.yyyy', 'dd-mm-yyyy', 'mon dd, yyyy'. If format is not specified, then the format will be taken from w2utils.settings.date_format.