I usually just use HL7 datetimes YYYYMMDDHHMMSSFFFF since it's mostly iso compatible and more importantly allows for varying levels of unknown values. You may

I usually just use HL7 datetimes YYYYMMDDHHMMSSFFFF since it's mostly iso compatible and more importantly allows for varying levels of unknown values. You may know what month something happened to you but you may not know the day in which case you would just write 201308 which is a perfectly valid date. You may know what day and what hour someone was born but you may not know the minutes or seconds (ex. 2013080506). By using it, there is no ambiguity because the year signals the order of values. It also sorts better than any of the other methods.