Hi all,

     I have this trigger written here. Now i wanna calculate the value of field Age_Of_ Insurance and assign it to the record that is entered.See the bold Line.

trigger Calculation  on Contact (before insert,before update)
{
   String  a='';
   String  month='';
   String  yr='';
   String  x='';
   String  dat='';
   integer m=0;
   integer d=0;
   integer z=0;
   integer i=0;
   integer yar=0;
   integer year=0;
   boolean variable=false;
   for (Contact c: trigger.new)
     {
        If(c.RecordTypeId  == '012O00000000Au3')                          //to make sure that trigger fires only in case of corporate contact record types.
        {
          a=c.SSN_for_Customer__c;                                       //assign ssid__c to a string variable 'a'. 
          if (a.length() != 8)
          {
           c.addError('Please enter SSID in yymmdd-x format');

          }
        else
        {
        month=a.substring(2,4);                                          //fetch string representing month in 'a'.
        dat=a.substring(4,6);                                            //fetch string representing date in 'a'.
        yr=a.substring(0,2);                                             //fetch string representing year in 'a'
        m=integer.valueof(month);                                        //CALCULATE INTEGER VALUE OF MONTH IN SSID
        yar=integer.valueof(yr);                                         //calculate integer value of yy in ssid.
        d=integer.valueof(dat);                                          //CALCULATE INTEGER VALUE OF DATE IN SSID
        x=a.substring(7,8);
        z=integer.valueof(x);                                            //CALCULATE INTEGER VALUE OF X (century).
            if(z==1 || z==2 || z==5 || z==6)
            {
               i=1900;                                                   // calculate century.
            }
            else
            {
               i=2000;
            } 
        year=i+yar;                                                      // to get value of year in YYYY format.
        Date myDate =date.newinstance(year, m, d);                       //create date outh of given integer values.
        Date today=System.today();                                       //fetch todays date from the system.
        integer numbermonths=myDate.monthsBetween(today);                //calculate number of months between two dates
                    if(numbermonths<0)
                    {
                     c.addError('Please enter SSID date lesser than current date in yymmdd-x format');

                    }
                    else
                    {

                      integer addmonth=numbermonths+6;                                 //add 6 to total number of months
                      double y=addmonth/12;                                            //get total number of years in between two dates
                      c.Age_Of_Insurance__c=y;                                         //assign this value to Age_Of _Insurance__c
                      break;
     }
     }
     }
     else
     {
     variable=True;
     }
   }
}

Now I wanna achieve this by writing the trigger after Insert and After Update.

How Can this be done?

Using the same code if I change The actions to after Insert it gives error.

Says

c.Age_Of_Insurance__c iis a read only field.

Whereas at back end its a number field.