C#.NET Calculate Age from DOB

A snippet of code I find comes up every now and then is how to calculate the age given a date of birth.  Obviously this could be re-written to take 2 dates and calculate the time between 2 dates rather than a specific date and now.

        private static int CalculateDay(DateTime dateOfBirth)
        {
            DateTime today = DateTime.Today;
            if (today.Day == dateOfBirth.Day)
            {
                return 0;
            }
 
            if (today.Day > dateOfBirth.Day)
            {
                return today.Day - dateOfBirth.Day;
            }
 
            // based on days in month
            switch(today.Month - 1)
            {
                case 1: 
                case 3: 
                case 5:
                case 7: 
                case 8: 
                case 10: 
                case 12:
                    return 31 - dateOfBirth.Day + today.Day;
                case 2:
                    if (today.Year % 4 == 0)
                    {
                        return 29 - dateOfBirth.Day + today.Day;
                    }
 
                    return 28 - dateOfBirth.Day + today.Day;
                default:
                    return 30 - dateOfBirth.Day + today.Day;
            }
        }
 
        private static int CalculateMonth(DateTime dateOfBirth)
        {
            DateTime today = DateTime.Today;
            int monthModifier = 0;
 
            // today is the birthday
            if (today.Day >= dateOfBirth.Day && today.Month == dateOfBirth.Month)
            {
                return 0;
            }
 
            // determine the month modifier
            if (today.Day >= dateOfBirth.Day && today.Month > dateOfBirth.Month)
            {
                monthModifier = 0;
            }
            else if (today.Day < dateOfBirth.Day && today.Month > dateOfBirth.Month)
            {
                monthModifier = -1;
            }
            else if (today.Day >= dateOfBirth.Day && today.Month < dateOfBirth.Month)
            {
                monthModifier = 12;
            }
            else if (today.Day < dateOfBirth.Day && today.Month <= dateOfBirth.Month)
            {
                monthModifier = 11;
            }
 
            // calculate number of months
            int month = today.Month - dateOfBirth.Month + monthModifier;
 
            if (month > 0)
            {
                return month;
            }
 
            return 0;
        }
 
        private static int CalculateYear(DateTime dateOfBirth)
        {
            DateTime today = DateTime.Today;
            int yearModifier = -1;
 
            // determine if current date is after the date of birth this year
            if ((today.Day >= dateOfBirth.Day && today.Month == dateOfBirth.Month) || (today.Month > dateOfBirth.Month))
            {
                yearModifier = 0;
            }
 
            // calculate the number of years
            return today.Year - dateOfBirth.Year + yearModifier;
        }

Usage would look something like:

     int month = CalculateMonth(dob);
     int year = CalculateYear(dob);
     int day = CalculateDay(dob);
 
     string age = "Age: " + year + " year(s) " + month + " month(s) " + day + " day(s)";