Error importing custom date fields when the day of month is above 12
In the CiviCRM engine the usuall date format is YYYYmmdd hhmm. You could try that date format. You could leave out the hhmm part. 

Explenation of the format
- YYYY = year in four digits (2014)
- mm (months in 2 digits, 01 for january)
- dd (day of month in 2 digits e.g. 21)