JAVA – How To Calculate An Age From A Date Of Birth (DOB)

The following code illustrates how to derive an age from a given DOB.

This example uses the DOB format YYYY-MM-DD.

This can be altered to any format by changing the substrings taken from the original DOB input.

String dob = “1984-09-20”;

//TAKE SUBSTRINGS OF THE DOB SO SPLIT OUT YEAR, MONTH AND DAY
//INTO SEPERATE VARIABLES
int yearDOB = Integer.parseInt(dob.substring(0, 4));
int monthDOB = Integer.parseInt(dob.substring(5, 7));
int dayDOB = Integer.parseInt(dob.substring(8, 10));

//CALCULATE THE CURRENT YEAR, MONTH AND DAY
//INTO SEPERATE VARIABLES
DateFormat dateFormat = new SimpleDateFormat(“yyyy”);
java.util.Date date = new java.util.Date();
int thisYear = Integer.parseInt(dateFormat.format(date));

dateFormat = new SimpleDateFormat(“MM”);
date = new java.util.Date();
int thisMonth = Integer.parseInt(dateFormat.format(date));

dateFormat = new SimpleDateFormat(“dd”);
date = new java.util.Date();
int thisDay = Integer.parseInt(dateFormat.format(date));

//CREATE AN AGE VARIABLE TO HOLD THE CALCULATED AGE
//TO START WILL – SET THE AGE EQUEL TO THE CURRENT YEAR MINUS THE YEAR
//OF THE DOB
int age = thisYear – yearDOB;

//IF THE CURRENT MONTH IS LESS THAN THE DOB MONTH
//THEN REDUCE THE DOB BY 1 AS THEY HAVE NOT HAD THEIR
//BIRTHDAY YET THIS YEAR
if(thisMonth < monthDOB){
age = age – 1;
}

//IF THE MONTH IN THE DOB IS EQUEL TO THE CURRENT MONTH
//THEN CHECK THE DAY TO FIND OUT IF THEY HAVE HAD THEIR
//BIRTHDAY YET. IF THE CURRENT DAY IS LESS THAN THE DAY OF THE DOB
//THEN REDUCE THE DOB BY 1 AS THEY HAVE NOT HAD THEIR
//BIRTHDAY YET THIS YEAR
if(thisMonth == monthDOB && thisDay < dayDOB){
age = age – 1;
}

//THE AGE VARIBALE WILL NOW CONTAIN THE CORRECT AGE
//DERIVED FROMTHE GIVEN DOB
System.out.println(age);

This process calculates the current year, month and date to work out if the DOB has already passed this year in order to accuratly establish the age.