validationrule for date as format yyyy-mm-dd in drools

Please give idea about this one..I have two textfield. User can select the date from calender or he can write the text to it.. when user enetr text we have to follow format yyyy-mm-dd: How i write the validation rule for date as a format yyyy-mm-dd. Please give sample code for this one. Its urgent

This is my bean class:

import java.text.SimpleDateFormat; import java.util.Date;

import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.Table; import javax.persistence.Temporal; import javax.persistence.TemporalType; import javax.persistence.Transient;

@Entity @Table(name = "DATSTMPEPOUTOF_SERVICE")

public class StampOutOfService extends BaseDataEntryFormEntity {

   private Date outofServiceStartDate;
   private Date outofServiceEndDate;
   private int lineYear=20;


   SimpleDateFormat df = new SimpleDateFormat("yyyy-mm-dd");

   @Column(name = "LINE_YEAR_FK", nullable = false)
   public int getlineYear() {
           return lineYear;
   }
   public void setlineYear(int lineYear) {
           this.lineYear = lineYear;
   }


   @Temporal (TemporalType.DATE)
   @Column (name = "START_OUT_OF_SERVICE",nullable = true)
   public Date getOutofServiceStartDate() {
           return outofServiceStartDate;
   }
   public void setOutofServiceStartDate(Date outofServiceStartDate) {

           this.outofServiceStartDate = outofServiceStartDate;
   }

   @Temporal (TemporalType.DATE)
   @Column (name = "END_OUT_OF_SERVICE",nullable = true)
   public Date getOutofServiceEndDate() {
           return outofServiceEndDate;
   }
   public void setOutofServiceEndDate(Date outofServiceEndDate) {
           this.outofServiceEndDate = outofServiceEndDate;
   }

}

my bean: for form creation:

                           <bean class="com.xxxxx.xxx"   p:caption="Out of Service Start Date" />
                           <bean class="com.xxxx.xxxx.dataentry.DataEntryFormFieldDate"

p:type="DATE" p:entityMapping="outofServiceEndDate" p:caption="Out of Service End Date" />

                   </list>
           </property>
           <property name="template">
                   <bean class="com.xxxx.xxxxx.dataentry.templates.HorizontalTableTemplate"

p:fixedColumns="1" />

Validation rule for date in drools:

rule "Stamping Out Of Service Start date should be valid date"

dialect "mvel"

when

droolsErrors : com.xxxx.xxxx.rules.beans.EntityErrors()

com.xxxx.xxxx.dataentry.beans.StampOutOfService (outofServiceStartDate !=null && outofServiceStartDate < df)

then droolsErrors.addError("outofServiceStartDate", "stmpOutOfServiceStartDateInvalid", "Please use the format yyyy-mm-dd");

end